You might want to check the amount of block sections you have.
A coaster requires 3 blocks for 2 trains (including chain lift if applicable). In a realistic scenario, this is to prevent a coaster leaving the station from crashing into a train if it has gotten stuck on a block brake.
Realistically, any coaster should have 2 block sections at the end of a coaster in the event the chain lift but some coasters do neglect this using the chain lift as a block and only having one block section at the end but only on rare occasions that the block run does not work.
So lets say you have a wooden coaster. If it got stuck on the block brake but there's not another block break behind it, then the coaster launched from the station would stop at the top of the chain lift. So the amount of block breaks would be equal to the number of cars you can have.
However, if you have a coaster without a chainlift such as torque launched coaster, then you would need an additional block break in addition to the block breaks needed for the amount of trains you'd like to have.
In laymans terms, A coaster can't successfully launch another car until the last car has PASSED a block section so if it is impossible to pass a block section without risking collision, then another block section is needed to have that many trains.
TL;DR you should have two block sections on any coaster at the end for 2 trains for any rollercoaster, not just one block break. Though it is optional to use the chain lift as a block break if you don't have enough room and have a complete disregard for safety in the event that both a block break run fails to launch the next car and the chain lift doesn't stop. I wonder what the odds of that are though. [wacky]
It's just another one of those advanced safety measures rollercoasters have to go by and another piece of realism added to the game.
If this has confused you at all, let me know. [wink]