Train fixes and balancing

- Slows trains to 90% of running speed
- Changes train impact damage to only take into account speed, not train length (otherwise you could insta kill people with a long enough train)
- Fixes a bug with towed cars ghosting through closed airlocks
- Stuns mobs who fall off a train that has blown up
- Fixes mobs standing/lying on vehicles
- Fixes exploding trains not decoupling correctly