Here's a picture:
Telling the metro where to go is as simple as picking all the platforms you'd like it to visit, then selecting the first one to end the loop.
I have sometimes ended lines with a very small loop, to increase line capacity. The train arrives in platform 1, and it's the end of the line so everyone gets off. The train keeps going in the same direction, out of the station, round a loop, and back into the station on platform 2 where it loads its passengers. Meanwhile, another train has come into platform 1. If the first train just went straight out of platform 1 in the normal way, the second train would have to wait slightly longer for the junction to be clear before it can come in. In the beta, I didn't find this very useful though, because it takes so long for train 1 to load that train 2 just ends up sat in the loop. The advantage: train 2's passengers get off the train sooner, and spend money for tram and bus tickets sooner, and after spending time at their destination will make another journey on your transport system sooner.
Frustratingly, trains always take the shortest route, which means the inner track of the loop, which means the line is crossing over itself here. In the beta, I found the line got stuck sometimes when a train was in the loop, another train blocks the crossing, and no-one can move. If the trains could take the outer line, this wouldn't be a problem!