TL;DR
Ability to put game into state where every input is streamed and later to resume game using this streamWhat ?
When player starts a new game or loads save file, game enters known state and is playable. When player does something and then game crashes (or is closed without saving) progress is lost. Logging all inputs could be used to restore game state to last recorded tick even in case of crash (well, restored state could also crash but this gives insight into reproduction). Input should be recorded before processing it.Game is already capable of streaming inputs (multiplayer), game is deterministic enough and can restore game state using input records (multiplayer's "catching up"). This would be "partial replay" as it would require initial state in form of map-gen-settings (when no prior save is available) or link to existing save file, info about version (in case of using old save in newer version), and series of input records. In case of crash, this file would be created (medium RAM requirement to hold all inputs in case of this) or could be continously updated during play. If crash happens that causes game to be not possible to save(consistency check during saving), at least there would be record of all inputs and you could replay it and during replay choose earlier time when game state is not corrupted and can be saved to recover progress. Full record could be used to investigate crashes.
By pointing into initial state, this replay data could be created and used even after map migration - replay would cover input records from only one game version (and set of mods)
Why ?
1/ Because of recurring issues with integrity checks with trains (that are not precise as to what caused it, only that state is corrupted)2/ as generic tool for crash reproduction
3/ to restore at least some progress after save