Railway/Train path finding

Before a train moves to a target (in this case, a Train stop), it calculates the best route based on the railway network at that time.

For calculation it uses a simple A*-algorithm: The pathfinder first builds a list of non-disabled stops that match the name in the schedule, then searches outward from both ends of the train at once, if applicable, in segments. A segment is an uninterrupted plain sequence of rails, with no intersections, stops, or signals (all of which define segment borders). The cost (distance) is calculated using the following weighting rules:

• Base cost for a block/segment is the length of the segment (linear grid length along the center of the rail).
• When the rail block is occupied by a train -> Add a penalty of 2 * length of the block divided by block distance from the start, so the far away occupied paths don't matter much.
• When the rail block is guarded by a rail signal set to red by the circuit network -> Add a penalty of 1000.
• When the path includes a train stop that is not the destination -> Add a penalty of 2000.
• When the path includes a train stop with a train stopped in it -> Add a penalty of 500.
• When the path includes a train stop with a train stopped in it that doesn't have other valid stops in its schedule -> Add a penalty of 1000.
• When the path includes a manually controlled stopped train -> Add a penalty of 2000.
• When the path includes a manually controlled stopped train without a passenger -> Add a penalty of 7000.
• When the path includes a train currently arriving to a train stop -> Add a penalty of 100.
• When the path includes a train currently arriving to a rail signal -> Add a penalty of 100.
• When the path includes a train currently waiting at a rail signal -> Add a penalty of 100 + 0.1 for every tick the train has already waited.
• When the path includes a train that doesn't have a path -> Add a penalty of 1000.

The route of a train is revalidated (and recalculated if it was invalid) in the following scenarios:

• A locomotive that is part of the train is rotated. The train is forced to make a new path.
• LuaTrain::recalculate_path() is called on the train by script. The train can be forced to recalculate its path regardless of validity.
• The train does not have a path and a train stop that is part of train's schedule gets renamed or created. The train is forced to recalculate its path.
• The train is pathing to a train stop that gets destroyed. The train is forced to make a new path.
• A rail which is part of the train's path gets destroyed.
  • If the rail is the destination of the path, the train is forced to recalculate its path.
• The train doesn't have a path and a rail gets created.
• A rail gets created and invalidates a signal (chain or regular) along the train's path. The train is forced to recalculate its path.
• A rail block changes and invalidates a signal (chain or regular) along the train's path. The train is forced to recalculate its path.
• A signal (chain or regular) gets destroyed or created.
• The train is set to go to a station using the "Go to stop" button in the train's GUI.
• The train's schedule is changed.
• The train is switched to automatic control when it was previously manually controlled.
• The train's braking force gets changed and the train is currently driving normally, arriving at a signal (chain or regular) or arriving at a station.
• The train is preparing to stop at a signal (chain or regular) that changes so that the train can now continue. The train is forced to recalculate its path.
• The train encounters a signal (chain or regular) it cant reserve and the train is not inside a chain signal block. The train is forced to recalculate its path.
• The train wants to depart from a train stop.
• The train has waited at a chain signal for a multiple of 5 seconds.
  • If the trains has waited for a multiple of 30 seconds or if multiple train stops with the name of the destination exist, the train is forced to recalculate its path.
• The train wants to depart from a signal (chain or regular) that it stopped at. The train is forced to recalculate its path.
• If the train doesn't have a path, the path is recalculated every 128 ticks.
• The train enters a new non-chain-signal block and can't reserve the next needed signal (chain or regular). The train is forced to recalculate its path.
• The train collides with something that is not a train (like a player). The train is forced to recalculate its path.
• The train is pathing to a train stop that gets disabled. The train is forced to recalculate its path.
• The train doesn't have a path and a train stop that is in its schedule gets enabled.

History

• 0.16.0:
  • The specific penalties can now be found in the utility constants, which allows mods to change them. (Undocumented)

• 0.15.0:
  • Train station adds 2000 tiles penalty when path finding, so trains try to avoid stations not related to their path.

• 0.11.17:
  • Increased the pathing penalty for non-moving train in manual mode from 200 to 1000.

• 0.11.13:
  • Stopped, manually controlled train adds additional penalty (related to train path finding) of 200 tiles to the block it occupies.

• 0.11.11:
  • The pathfinding is based on penalties for blocked segments now. For trains waiting in station, the more remaining time in the station, the bigger penalty.

See also

• Train pathfinding as of version 0.16 (Code)
• FFF #68
• Railway