I am not that familiar with railroad pensions, but I believe the question is just to determine if it is disability or not.
If you are younger then the minimum retirement age then you would only receive it because of disability. If you are older than that age then it must be a pension. The difference is how it is reported on the 1040 tax return.
A regular 1099-R has a special code to indicate disability but a RRB does not, so the only way to know if it is disability or not is if you are getting retirement benefits before retirement age.