Callback: Vehicle Start/stop check

Vehicle Start/stop check (31)

This callback is called whenever a player (or the AI) tries to start or stop a vehicle. This is mainly useful for preventing vehicles from leaving the depot unless a given condition is met. To check whether a vehicle is being started or stopped, check bit 1 of variable B2. If set, the vehicle is currently stopped and if clear, the vehicle is moving (or at least, not stopped manually).

GRFv7 For GRF version 7 and below the valid return values are:

00..FE	Display error message using D0xx (D000..D0FE) text as second line.
FF	Start/stop action shall succeed.
100..1FE	Display error message using D0xx (D100..D1FE) text as second line.
200..2FE	Display error message using D0xx (D200..D2FE) text as second line.
300..3FE	Display error message using D0xx (D300..D3FE) text as second line.

GRFv8 For GRF version 8 and above the valid return values are:

00..3FF	Display error message using D0xx (D000..D3FF) text as second line.
400	Start/stop action shall succeed.

The callback is always used when defined, no bit in the action 0 property needs to be set to activate it.