Player (no armor) vs 10 km/h train - you will lose your HP in ~1.5 second
Player (no armor) vs 20 km/h train - you will survive if you will move out of rails almost immediately after being hit
The speed of 80 km/h and lower is safe for cars (this means - the car will not be destroyed but will lose some HP) and the tank would survive even higher speeds.
If you have a power armor with few shields then you would survive at almost any speed.
Trains slowed to 10-20 km/h are approaching very slowly, so you would be able to notice it even with small zoom and run away.