I think I figured it out. It looks like you have to place the signal even if it's pointing in the wrong direction. Then you click on the signal once the game shows a "2 arrow" icon. This will reverse the direction of the track. Then you hold Shift and click to create a "stop sign" so the trains don't collide.
if you have a pc with a mouse? i hold down shift and spin the wheel on the mouse! that changes the directional signal direction, you see a little red thing/circle in front of the arrow. the direction arrows work as a stop sign too. look on you tube for a fellow named "Adekyn" he has some good tutitorials on placing signals and how warehouses work, and assigning trains to platforms at stations and what track to use. ect... some good info. heres a link to the signal one https://www.youtube.com/watch?v=jy79xblbZ8s
how to merge lines and set signals
heres a better one https://www.youtube.com/watch?v=iIOAcOh4t4Y