Both will be fine. Trains are less likely to crash because they run on tracks. Of course train crashes do happen but it's rare. You can also
Intercity trains always have wheelchair spaces alongside a carriage with space for larger luggage which you can usually use - if you know who is running the service, you can usually look up which carriage it will be in, or you can sit in the corridor, or you can take a small one and wedge it between seats.
Trains don't have seatbelts for passengers, and if it was considered safer, they would. So I think the issue is that in a train crash if you're in the affected area your chances are not great anyway and if you're not, you're usually fine. So it would make no difference.
Coaches you can usually take a car seat onto, but it's a pain. At 8 months, perhaps she still fits into a carry type seat? You could take one which goes on a pushchair and use that, folding the pushchair to put under the coach. But sometimes there is only a lap belt, and most infant seats don't fit with a lap belt.
I would still say that coaches are less likely to crash than cars. Firstly because they are bigger, so other vehicles are more likely to avoid them. Secondly as they are bigger they decelerate more slowly even in the event of a collision. It is the sudden deceleration which is dangerous in a car accident because the occupants tend to keep moving until stopped by a seat belt or the road. In a coach crash you don't have the same issue. If you look at news coverage of fatal coach crashes they don't tend to be road accidents, they tend to be rural areas and the coach rolls into a ditch or something like that. In the very rare event of a collision, the passengers are usually fine because other vehicles are lower than the coach passengers, and because coaches are so long that if they smash at the front people from about row 3 are shaken but fine, even children.
For general restraint you could use a sling but if you use a sling and a coach with a seatbelt, do not put the seatbelt over both of you, only yourself.
I can't give you any exact figures of probability, but I'd say that you're about even in danger from coaches and trains, both extremely low, and I'd choose whichever option is more convenient or cost effective.