Time needed for superman to stop the train

[zhenyazh]

hi,
can some one give me an outline of how to solve the following question?

thanks

Superman (mass=84.0 kg) must fly into a train (mass = 18079 kg) traveling at 75.0 km/hr to stop it.
Calculate the minimum time Superman must take to stop the train, if the passengers experience an average horizontal force of 0.420 their own weight.

 

[willem2]

If the train is stopped, the passengers have also stopped.
Use F=ma on a passenger