MC 1.7.2.
Railcraft 9.1.0.0(for 1.7.2)
buildcraft-6.0.16
forestry 2.4.1.28(for 1.7.2)
Engines from Railcraft do not give energy to forestry Carpenter. When i switch Railcraft engine to Buildcraft engine then it works okay. Railcraft engine produces energy. If located near Carpenter energy is not transferred. If using buildcraft pipes (visually energy is transferred) energy is not transferred. Forestry says it is compatible. I asked people around and they said it should work. I checked a lot of videos and tutorials and everything is set okay. (with buildcraft engines it works).

Somebody told this should a problem of compatibility of RailCraft and BuildCraft 6 compatibility

Yes, Railcraft does not support Buildcraft 6. Some things might work, but Railcraft is currently using Buildcraft 5.

This would be a Buildcraft bug since I haven't changed anything on my end.

Report it to BC.