At the risk of being too clever, here's the website. Yes SSA counts, to a certain level, if you have both, in calculating Tier 1 benefits, not Tier II though (ain't no Tier II contribution to SSA).
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www.rrb.gov]
The case of Congress stealing part of our early RRB contributions in the "reforms" of the late 80's and 90's, because one actually later got a job with another type of good pension, is a problem, and they do reduce Tier I and Tier II for that.
So if you have any non-military or public safety public pensions, for say teaching, or such, you need to figure for that "offset" (aka-essentially a "theft" of credit for a portion of your previous RRB contribtutions, which they had no trouble taking from you and your employing RR). Don't feel too bad, as I noted, they didn't just screw former rails, they screwed a few million teachers and other non-rails with this too, as the same "offset" provisions apply to SSI only recipients. Be thankful that Congress only stole part of it though, as it's only a partial "offset".
But, you can have most questions answered, using this site, and calulate the effects of most of the permuatations by asking for a review, based on alternative scenarios, and an updated statement of benefits from a local office in about two working days, which includes most recent SSI inputs. Nice folks at the Covina RRB Office are very helpful.