A deer would be unlikely to jump into a train of cars. They usually get caught by cars that have space in front of them. I know you are just giving a single example, but anything that applies to an actual train would apply to a virtual train as well. I don't understand why not having a physical latch between cars makes any real difference.

A deer doesn't affect a real train that much due to the mass difference. In case of cars though they cause serious damage. If the following cars don't keep a distance they end up immediately crashing into the first one, multiplying the damage.

I don't have much experience with deer behaviour, but kangaroos will happily jump right into anything moving. They will even hit the side of your car sometimes (which they should've obviously seen coming)