I have question about using Kurzstrecke ticket to travel from Berlin to Potsdam. As I see it's the most economic ticket plan for stay in Berlin with one planned visit to Potsdam. So I'd have to buy A+B ticket for Berlin and 2 Kurzstrecke tickets for 1,40 Euro each.
But, how to validate Kurzstrecke ticket in S-Bahn? Can I do it during the passage? Or I need to go out, validate the ticket and wait for the next S-Bahn train?
I think you want an extension ticket, but see below.
Short Trip Tickets requiring validation must be validated on starting travel.
The idea is that you need to have one ticket that's valid for your whole trip.
Technically, on the journey out, you may step off the train at the last zone B station, validate the ticket, and step back onto the same train. But unless you know exactly where the validator is on the platform and position yourself accordingly, you'll be stranded until the next train comes.
On the return journey, you need to validate your single ticket at the start of the journey, and you're supposed to step off and back on at the first zone B station. Technically, if a ticket inspector happens to step on at that point and realizes you did not have a ticket for the whole journey, the rules state that they should fine you.
There is a different ticket, the extension ticket (Anschlussfahrschein), which is meant for use in combination with an ordinary A+B ticket to prolong one trip into zone C (or start in zone C and continue through zone B and optionally A). This ticket is explicitly not allowed in combination with the A+B WelcomeCard or with the A+B CityTourCard. I can't find a definitive reference regarding its validity in combination with a daily, weekly or longer ticket. The official FAQ only explicitly excludes “WelcomeCards, CityTourCards, Schulklassen-Tickets und FamilienPass-Monatskarten”, so I surmise that it is valid in combination with other passes, but please confirm with a ticket office first (and report back!).
I don't think you can validate the ticket on the train itself. The German page for Kurzstrecken-Tickets states:
"Kurzstreckenfahrscheine that need to be validated have to be validated at the beginning of the journey"
Even though BVG has English pages there doesn't seem to be an equivalent for that page in English.
If this is too vague, you can validate the ticket on the S-Bahn platform in Wannsee which is three stops from Potsdam. The stop may be long enough to hop off the train, validate the Kurzstrecken-Ticket and then hop back on the same train. If you miss it, you can always use the next train.
Returning from Potsdam is easier as you just validate the Kurzstrecken-Ticket in Potsdam on the platform and then use your Berlin A-B ticket from Wannsee onwards (assuming the A-B ticket is one for one or multiple days)
So I don't see a problem here, you can still try to ask the staff on the platform, but they don't always speak good English.
2019:
When you start your journey with any valid AB or BC ticket, you stamp the extension-ticket thus creating a valid ABC ticket.
The Kurzstrecken ticket may not be used for this purpose (different price).
(In 2012 this may have been possible, but for many years now no longer is)
The Kurzstrecken (short distance) may only be used for 3 U/S-Bahn or 6 Bus/Tram stations.
