BRASS as others have mentioned is still part of the Railways Pension Scheme, albeit nowhere near as good as it once was.
It's an Additional Voluntarily Contribution that buys units in an investment fund, there are a mix of funds to choose from although most people will be in the default target retirement age funds which reduces risk as the expected retirement age is approached.
BRASS used to be an easy choice before 2009 as you could, at a rate of 12 to 1, use BRASS funds to buy additional index linked annual pension (caveat of the additional income dies with you iirc) or take it as a lump sum at retirement. Note:- The only thing you can do at the moment is commute annual index linked pension to lump sum at 12 to 1 and BRASS funds can either be taken as a lump sum or used to buy an annuity from an insurance company.
For those paying in earlier, unsure of the dates, and protected members there were matched contributions as well so if you paid in 5% the company would match that up to a certain point. Essentially it was free pay rise for those who started paying into BRASS at the right time.
If you started paying in after 2009, non-protected member etc, it's just a run of the mill additional voluntary contribution scheme like you find with many other companies.
Positive thing about paying into BRASS is that it's simple and comes off payroll before National Insurance so could save you 12% if coming from income within the basic rate band or 2% if coming from income that would be taxed at a higher rate. Note:- Income Tax relief is the same for any type of pension public, private, defined benefit or defined contribution.
Negatives:- Limited investment fund choices, access to BRASS tied to main scheme retirement age therefore not accessible unless you pull the trigger and activate your main RPS annual pension which is subject to actuarial reduction. This makes retirement planning a bit more difficult if you don't plan on wanting to retire at whatever age it will be by the time you get there e.g. 68 for younger folk.