In terms of rights for the whole flight+train combination, they are quite limited. The Interpretative Guidelines for EC261 tell us in no uncertain terms:
6. COMPENSATION, REIMBURSEMENT, RE-ROUTING AND CARE IN THE CASE OF MULTIMODAL JOURNEYS
Multimodal journeys involving more than one mode of transport under a single transport contract (e.g. a journey by rail and air sold as a single journey) are not covered as such under the Regulation, nor are they covered by any Union legislation on passenger rights in other modes of transport. If a passenger misses a flight because of a delayed train, he or she would only benefit from the rights to compensation and assistance granted by Regulation (EC) No 1371/2007 of the European Parliament and of the Council (54) in relation to the rail journey, and then only if the passenger was delayed by more than 60 minutes at the destination (55). By the same token, other provisions would apply in the case of a flight missed following a delayed ship or coach journey in the context of a single contract of carriage (56). However, organisers of packages may be liable under Directive 90/314/EEC or Directive (EU) 2015/2302 also for the missed flights and the impact on the package as a whole if the multimodal journey forms part of a combination with other travel services, e.g. accommodation.
So if the train does not run, then it's not covered by EC261 but by 2021/782. The corresponding rights are explained on the EC's Rail passenger rights website:
If you are delayed due to a cancelled train, meaning you would arrive at your final destination with a delay of more than 60 minutes, you have the right to choose between:
a refund of your ticket within 30 days – this may be a full or partial refund (covering the part of the journey not made), and a return journey to your initial point of departure, if, the delay due to the cancelled train prevents you from fulfilling the purpose of your trip, or
continuing or re-routing your journey under comparable conditions to reach your final destination at the earliest opportunity, at no additional cost, or
continuing or re-routing your journey at a later date of your choosing under comparable conditions, at no additional cost. This includes alternative transport to get you to your final destination when the train is blocked and the service is suspended.
Assistance and alternative travel arrangements
In addition, you are entitled to assistance in the form of meals and refreshments (proportionate to the waiting time). You also have a right to a hotel or other accomodation - where and when physically possible - if you have to stay overnight or an additional stay is necessary due to the cancelled train. This includes a right to transport between the hotel/accommodation and the station.
In the event of a cancellation, the railway company may allow you, upon request, to make your own alternative travel arrangements with other transport service providers (including rail and other modes of transport) which would enable you to reach your final destination under comparable conditions. In this case, the railway company will reimburse the costs you incurred.
However, if the railway company does not inform you of the available re-routing options within 100 minutes of the scheduled departure of the cancelled train, you are entitled to make your own alternative travel arrangements without the agreement of the railway company with other public transport services i.e. rail, coach or bus. The railway company must then reimburse the necessary, appropriate and reasonable costs that you incurred making these alternative arrangements.
Even if you decide to continue your journey as planned or accept alternative transport to your destination, you may also be entitled to compensation if your arrival is delayed by 60 minutes or more. Note that national rules in some EU countries may grant more favourable re-routing conditions to passengers.
There's also compensation of 25% or 50% of the ticket price if the delay in arriving at your destination is over 1 or 2 hours, respectively. They will try to invoke extraordinary circumstances, but strikes by their own personnel are explicitly excluded of extraordinary circumstances.
Note however that some trains may be exempted. "Domestic" trains in Germany are in scope, but "Urban, suburban and regional services" may not be.
Still, I would be extremely surprised if you managed to get any practical help from DB in this situation. Your best bets are probably:
- Check if the airline is able to reroute you to an airport closer to your destination or somehow more practical
- If the strike does not affect ALL trains, and you can know in advance which ones still run, try to move your booking to a train which runs
- If you can drive, to rent a car
- If you can't or prefer not to, to use a long-distance ride-sharing service such as BlaBlaCar (no idea if there are local equivalents in Germany)
You should be able to get either:
- a refund of the part of your ticket for the train travel
- or a refund of your costs to get to your destination by other means + compensation if that gets you there over an hour late
But it will probably take quite a while, and expect some pushback.