How do I add more coaster trains?

I can never add more coaster trains to my roller coasters I don't understand why, the arrows never are shaded so I can't click them and whenever I try to change the number it reverts straight back to one. I'm not sure if the issue is block brakes because I've tried adding them to the coaster, I've done two right before the coaster ends, I've done block brakes in the middle or at the beginning of the track but it doesn't seem to show up.

look here, this might be an explanation to your question:

TUTORIAL: Multiple trains - Easiest way to add 2 or 3 trains on a single coaster!

Thanks a lot exactly what I needed!