I'm not absolutely certain what requires a railroad to be part of RRB or not, for example ACE isn't. One thing is true: if an operator takes over an RR operation which has been paying into RRB, then that operator must continue to do so.

> Our engineers who where retired RR would abstain from this activity.

Not quite sure what you mean here, but once an RRB-covered employee retires, he/she may not seek employment in the craft in which they were employed before retiring. IOW. a retired engineer can't work as an engineer, although he/she can as a brakeman. Why the RRB stipulates this is a bit of a mystery because it negates all that accumulated work experience and skill, further preventing it from being passed on to the next generation. There are also issues of post retirement employment contributing to SSI, which can cause complete and permanent cancellation of an RRB pension.