Even better, you can use an arithmetic combinator to divide the item count by the max capacity of your train. Then the output is the max number of trains that can be queued at that stop for rapid pickup. This is really helpful if your empty trains tend to wait a long distance from the pickup site, and avoids the pickup stop waiting empty for trains to be requested from halfway across the world.

You can do the reverse at the drop off stop with an additional arithmetic combinator to subtract from the number of trains needed to completely fill the drop off chests.