[18:32:39] [Server thread/WARN] [FML]: Railcraft loaded a new chunk [5, 3] in dimension 0 (overworld) while populating chunk [5, 1], causing cascading worldgen lag.
[18:32:39] [Server thread/WARN] [FML]: Please report this to the mod's issue tracker. This log can be disabled in the Forge config.
[18:32:39] [Server thread/WARN] [FML]: Railcraft loaded a new chunk [5, 5] in dimension 0 (overworld) while populating chunk [5, 3], causing cascading worldgen lag.
[18:32:39] [Server thread/WARN] [FML]: Please report this to the mod's issue tracker. This log can be disabled in the Forge config.
[18:32:39] [Server thread/WARN] [FML]: Railcraft loaded a new chunk [6, 6] in dimension 0 (overworld) while populating chunk [5, 5], causing cascading worldgen lag.
[18:32:39] [Server thread/WARN] [FML]: Please report this to the mod's issue tracker. This log can be disabled in the Forge config.
[18:32:39] [Server thread/WARN] [FML]: Railcraft loaded a new chunk [7, 7] in dimension 0 (overworld) while populating chunk [6, 6], causing cascading worldgen lag.
[18:32:39] [Server thread/WARN] [FML]: Please report this to the mod's issue tracker. This log can be disabled in the Forge config.

Is this problem fixed? I constantly get these messages (1.12.2). What to do with it?

leave it. it is just a message with no effect.

Is there an option to fix this?

log does not help. a map showing extra chunks generated helps.

I do not quite understand how I can help you?

We will do some debug in the future. If you have a report that Railcraft caused major snowball effect in chunk loading, let us know.

@liach

https://pastebin.com/NdzuVUv3
This is just a part of the logs, and such records are full.
Version railcraft: Railcraft 12.0.0-beta-5 - MC 1.12.2
(In the previous version, everything was the same.)

This is done with debug run in ide. It is fairly complex to newcomers and require good devices. I will test in an ide later.

Fixed the one in ore mine.

Not fixed after testing. Needs a throughout check.