Three identical train cars, coupled together, are rolling east at speed v0. a fourth car traveling east at 2v0 catches up with the three and couples to make a four-car train. a moment later, the train cars hit a fifth car that was at rest on the tracks, and it couples to make a five-car train.

Towards East

Explanation:

As we know that whole system is moving without any external force on it

So here we can apply momentum conservation

Now we have

so above is the speed of four coupled train

Now when this is coupled with fifth stationary car then we have

Towards East