Callback: Can wagon be attached
Can wagon be attached (1D)
This callback is called for the train engine (i.e. from the engine's action 3) when attaching a new vehicle to the current train and determines whether it may in fact be attached to the train. The callback is always used when defined, no bit in the action 0 property needs to be set to activate it.
You may use VarAction2s to check variables 40+x and 80+x. For var. action 2 type 81 (vehicle), they refer to variables of the wagon that is to be attached, not the engine it is being attached to, so you can check the wagon's properties like the cargo type to see whether it is appropriate for the given train engine.
2.5 Var. action 2 type 82 (engine) refers to the engine the wagon is being attached to, starting from version TTDPatch 2.0.1 alpha 46. This allows you to for example find out the length of the train it is being attached to. In earlier patch versions, it refers to the engine of the source consist (or the wagon if the wagon is not attached to anything else), which was not really helpful.
GRFv7 For GRF version 7 and below the valid return values are:
| Return | Meaning |
|---|---|
| 00..FC | Disallow attaching and use the D0xx (D000..D0FC) text ID as second line of an error message, |
| FD | Disallow attaching with the standard message (incompatible rail types) |
| FE | Allow attaching[1] |
| FF | Allow attaching if the rail types match (default) |
GRFv8 For GRF version 8 and above the valid return values are:
| Return | Meaning |
|---|---|
| 00..3FF | Disallow attaching and use the D0xx (D000..D3FF) text ID as second line of an error message, |
| 400 | Allow attaching if the rail types match (default) |
| 401 | Allow attaching[1] |
| 402 | Disallow attaching with the standard message (incompatible rail types) |
