Callback: Vehicle Start/stop check
From GRFSpecs
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).
GRFv≤7 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. |
GRFv≥8 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. |
| 40F |  15  Display error message using text ID from register 0x100. (not limited to D0xx texts)
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The callback is always used when defined, no bit in the action 0 property needs to be set to activate it.
