Can someone explain how to do
a railroad car of 2000 kg coasting at 3.0 m/s overtakes and locks together with an identical car coasting on the same track in the same direction at 2.0 m/s. what is the speed of the cars after they lock together.

Both cars are canceling out the energy of each other. The energy that is left over because one car has a bigger kinetic energy is the energy that is going to move BOTH the cars.

First Car : 0.5 * 2000kg * 3m/s^2 = 10.24 KJ

Second Car : 0.5 * 2000kg * 2m/s^2 = 4 KJ

Leftover Energy : 10.24 KJ - 4 KJ = 6.24 KJ

Movement of BOTH cars :
6.24 = 4000kg * v^2
v =  √(6.24 / 4000) =0.04 m/s = 4 cm/s

It's because 3 if it was 10 it wouldn't look right do it goes up to 4d