My game crashes upon start!
This is probably related to the stupid DRM within Railcraft (why would someone even do shit like that???). To fix this, use Oracle Java 8 Update 321, which is guaranteed to work (some older versions and some newer might work as well, but they might also run into the same problem. Update 321 is guaranteed to work!) A Oracle Java version archive is available here (the JRE version is enough - you do not need the JDK): https://www.oracle.com/de/java/technologies/javase/javase8u211-later-archive-downloads.html
My game still crashes upon start!
Most likely you are using PrismLauncher or MultiMC or some other 3rd party launcher. To fix this in PrismLauncher and MultiMC, click on "Edit Instance" while the modpack is selected. Then click on "Version" on the left and disable the "Forge" entry in the list (uncheck the checkbox field). Forge together with some miscellaneous fixes and overrides is already shipped within this modpack, so it should not be added again. The official TechnicLauncher does not have this problem!