What happens if lots of trains go in one direction in a section that uses physical tokens, and one station then runs out of tokens?

As may be deduced from the description of token block systems above, there is a problem if the traffic is not symmetric in both directions between the two stations at the ends of a block section. Eventually, one of the stations will run out of tokens to dispense to trains. This condition is known, not surprisingly, as a token exhausted condition, and results in all traffic in that direction being suspended until tokens (recovered from the other station) can be refilled in the empty block instrument.

In order to avoid this situation, every so often (depending on the traffic), tokens are taken from the station that is receiving an excess of them and returned to the other one (which is dispensing more than it receives); this is known as token balancing.

Source - IFRCA.org