For most journeys, only one train company can cover that route, so there's no comparison involved. For your journey from London to Newcastle-upon-Tyne, the only option is to use East Coast.
When there are multiple companies, you can check using "National Rail Enquiries" which can search all the companies. A good example is London-Birmingham, for which there are three options (London Midland from Euston to New Street, Chilterns from Marylebone to Moor Street and Virgin from Euston to New Street).
If you search for travel today, and you see more than one price, all labelled as the same type of ticket (e.g. "Off-Peak" or "Anytime") then there are different companies. Click on the "other tickets" section and then click the + beside Off-Peak or Anytime hover over the word "Off-Peak" on the list that appears - you will see that some of these are "ROUTE OF TICKET ANY PERMITTED" which is a ticket that can use any company, and others are "ROUTE OF TICKET LDN MIDLAND ONLY" or "ROUTE OF TICKET CHILTERN ONLY" or "ROUTE OF TICKET VIA HIGH WYCOMBE" (these are Chilterns Trains).
The "ANY PERMITTED" ticket is the price set by the most expensive company, and then the cheaper companies will have either "(name of company) ONLY" tickets, or, if they use a different route to get there, "VIA (some station that only they go through)".
There are three types of tickets:
"Anytime", which allow you to take any train at all if they are ANY PERMITTED or any train with the particular operator or on the particular route if they have a restricted routing.
"Off-Peak", which allow you to take any train, except "Peak" trains. Peak trains are usually any weekday train leaving its first station before 9:30 am (some very early morning trains may allow off-peak tickets; usually ones arriving before 7 am; for some very long journeys, slightly later trains may count as Off-Peak) and some trains leaving London between 15:01 and 18:44. A train is always either Peak or Off-Peak, so even if you get on at 11:00, if the train started at 9:20, then it's a Peak train and you need an Anytime ticket. Weekends and bank holidays are Off-Peak all day. For some journeys there are both Off-Peak and Super Off-Peak trains - generally, Peak would get you to work for a full day, Off-Peak will get you to you work for lunchtime, and Super-Off-Peak isn't useful for work at all.
Both Anytime and Off-Peak tickets can be bought at any time, including at the station directly before the train leaves. If you're travelling from a station where it's permitted to board a train without a ticket (usually because there is no ticket office, or the ticket office is unstaffed at the time of boarding), then you can buy the tickets on board train, or sometimes at the destination station.
For very short journeys, such as most commuter trains, these are the only tickets available; for longer journeys, there is a third type of ticket available:
- "Advance" tickets. These are cheaper than the two flexible ticket types, but have to be purchased in advance, no later than the day before travel. They are normally available twelve weeks before, but may be delayed if there is some uncertainty what the timetable will be twelve weeks ahead (this often applies to weekend tickets due to engineering works, and also applies more widely in the run up to the timetable changes in May and December). These tickets are attached to a reservation, and you can only travel on the train that you have the reserved seat on.
There are a number of different prices for Advance tickets on a particular train (there can be ten or more prices for some trains); the ticketing system sells the cheapest tickets until they sell out and then starts on the next-cheapest. Once all the Advance tickets are sold for a particular train, the only way to catch that train is a flexible ticket. For this reason you should always try to book as far in advance (up to the maximum of twelve weeks) as you possibly can. Unlike airline tickets, the prices will not fall closer to the time of departure, so there is never a good reason to delay purchase.
Advance tickets are always singles, so if you want to go both ways, then just book two singles. Off-Peak and Anytime tickets are always cheaper to get a return than two singles, but for many journeys the return is exactly one pound cheaper than two singles, so if you can take an Anytime in one direction and an Off-Peak in the other, then that's cheaper than the Anytime Return. For short journeys, there may be a Day Return ticket, which will be a lot cheaper than two singles. These are generally only on the short journeys where Advance tickets are not available.
Single tickets are always only for the specified day (chosen when you book) but for Off-Peak and Anytime Return tickets (not Day Returns), the outward half is day you specified, and the return half is for any day within thirty days of the outward journey. This additional flexibility may make a period return better than two singles in some circumstances.
There are some unusual return tickets available, which are often very good deals, e.g. the (Virgin Trains only) Super-Off-Peak One Day Travelcard, which is a booked-Advance ticket to London combined with a (same-day) Anytime ticket back from London at about the price of two Advance tickets. This particular ticket is only available on Saturdays and Bank Holiday Mondays; train companies bring in and scrap all sorts of unusual promotional tickets all the time. Train booking systems will offer you the various discounted options except for a few special deals mentioned in the footnote.
There are a number of railcards. A railcard normally entitles you to a discount of around a third on most train tickets bought over the course of a year, and they cost up to £30. The four main railcards are Young Person (16-25), Senior (60 or over), Family (issued to two named adults, journeys must be a minimum of one of the named adults and one child to get discount, maximum of four adults and four children) and Two Together (two named adults, must both be travelling together to get the discount). There are also HM Forces and Disabled Person railcards.
The main restriction on most railcards is to prevent them being used for commuting to work, either by restricting them to journeys starting after 0930 on workdays or by having a minimum price (currently £12) for journeys before 0930 on workdays. The exact rules change frequently and vary from railcard to railcard, so check the individual railcard you're interested in carefully. Some railcards allow First Class, others don't - again, check.
There are also local railcards, which offer (sometimes very large) discounts to people living in particular areas on their local rail services. These are mostly in tourist areas so that the standard price can be elevated (for the tourists) while local residents can still use the rail service. You have to produce evidence that you live in the right area to get one of these.
A sort-of local railcard is the Network Railcard which is for non-commuter fares in the London commuter belt, and is available to anyone, regardless of where you live. London commuter fares are normally higher (per-mile) than other fares in the UK, and the Network Railcard is intended to bring them back into line for leisure journeys.
Children (under 16) get discounted child fares, normally half the adult price. For teens under 16, it's recommended to carry some form of proof of age when travelling on a child ticket. Under fives travel for free with a fare-paying passenger, but you can't reserve them a seat; if you want a reservation, buy a child ticket.
When buying tickets online, you can indicate which railcard you have and it will incorporate the discounts into the prices. The ticket will indicate that it is for a passenger with a railcard, and you must have the physical railcard with you when you travel.
Season tickets are available in two types: weekly (green) which do not require a photocard and period (yellow) which do require a photocard, and are available for anything from one month to 12. They are the equivalent of an unlimited number of Anytime tickets along their journey route (in either direction). The only variations are child/adult and first/standard class. For some (longer) journeys, only period seasons are available. Railcards can't be used with season tickets.
These are mainly aimed at people who regularly commute along the same line, but can be useful for some other purposes (e.g. people attending a conference who are staying in a hotel a distance away might get a season ticket).
Where to buy from
All tickets are entered into a single booking system, which all the websites have access to; there's no price reason to choose one site over another. Some charge a booking fee, some don't, some are better about overseas purchase, some are worse, some are easier to find the cheap tickets on, etc. All tickets should be available from all ticket offices at stations, but I'd generally recommend going to a larger station if you want anything slightly complicated.
When you buy a ticket online, you can either have it posted to you for a fee, or you can collect it from a machine in a station (the smaller stations don't have machines). To collect a ticket, you need the credit card you used to pay for the ticket online, and the booking reference; the website should send you a booking reference. Some travel agencies have ticket-printing machines, but they will generally only print tickets that are bought through the agency, usually with a mark-up.
You can also buy tickets from a machine at a station. These machines are aimed at passengers intending to travel immediately from that station and often will only sell a small range of journeys for immediate travel. Very few will sell advance tickets. You can still use the machine to collect tickets you've bought online, though, including ones you bought using an app while standing in front of the machine.
Sometimes, train companies offer discounts for buying tickets for their trains from their own websites, so it's often worth checking the website for the company you're travelling on. These are the normal tickets sold elsewhere, just with an extra 10-15% knocked off. Also, some unusual ticket types are not available on all rail companies.
Footnote: unusual ticket types that exist but are hard to find on the booking systems:
- SailRail - these are discounted tickets to Holyhead, Fishguard, Birkenhead or Stranraer that are bought jointly with a ferry ticket to Ireland, or to Heysham with a ferry to the Isle of Man
- Dutch Flyer - these are discounted tickets to Harwich that are bought jointly with a ferry ticket to Holland
- CIV - these are discounted flexible tickets to London that require you to have a Eurostar ticket (they don't have to be bought jointly), and which entitle you to be put on the next available Eurostar if the UK train was delayed resulting in you missing your booked Eurostar. Note that Advance CIV tickets also exist (at the regular Advance price), but can only be bought direct from Eurostar, whereas flexible CIV tickets are discounted and from normal flexible tickets and can be brought from other suppliers.
- Sleeper berths - there are three sleeper trains that leave London each day (one to Penzance, one to Edinburgh and Glasgow, and one to the Highlands of Scotland). There are supplements required when you book to get a berth in the sleeper, and it's very easy to end up with a seat-only ticket unless you watch what you're doing.
Note also that various cities have combined ticketing for buses, trams, metros and trains, so you can sometimes get a city-wide day travel card for a similar price to a simple train ticket, e.g. in London or Manchester. These travelcards are not shown on the normal train ticketing engines.
Most short-distance trains only have one class, ie Standard Class (used to be known as second-class). Longer distance trains often have two classes, Standard Class and First Class. Generally speaking, Standard Class is 2+2 seating (two seats either side of the aisle) and First Class is 2+1 (two seats one side, one seat the other). This means the First Class seats are considerably wider, and you have the choice of a table for four or a table for two; Standard only offers tables for four. There is usually more legroom in First Class, and often complimentary WiFi, and complimentary tea and coffee. Some First Class journeys may include a complimentary meal, but that's fairly unusual.
Some (larger) stations have a First Class lounge, which is a waiting room with better quality seats (upholstered rather than plastic), WiFi, complimentary snacks, soft drinks and hot drinks, and sometimes the ability to purchase alcoholic drinks. They're not really a match for airline First Class lounges, but they beat the heck out of the plastic or metal seats on the rest of the station, or (Euston) having to stand while waiting.
First Class tickets are sold in the same three categories as Standard Class tickets. They generally range from about 50% more than Standard to twice the price of Standard. Very occasionally, the cheapest Advance Standard tickets sell out and there are First Class Advance tickets cheaper than flexible Standard tickets. The WiFi and tea and coffee in the station and on board train can add up to a similar price to the differential between the Standard Advance and First Class Advance ticket, so the First Class can work out as a saving if you would be buying those services anyway.
Try looking at Seat61 which has an excellent page with more information on this subject.