3 to 5 years, or last two employers is standard, but I can't answer for particular employers (I have got as far as being offered a job - not a signaller - by Network Rail, but it didn't happen for various reasons, and another with a TOC - again not a driver - which didn't happen because covid. I can't now remember exactly what they asked for, and both were a few years ago now, so it's possible things have changed.)
I think in both cases there was more than one employer in that timescale. Not sure if they would want some sort of second reference, either from an earlier job or a personal / character reference if you've been with the same employer for more than 5 years.
Having said that, most employers will only confirm dates of employment rather than give a detailed reference that might lead to an argument (it is an urban myth that employers are 'not allowed to give a bad reference', but if they say anything that's untrue / malicious, they can get taken to court, so many avoid the risk by not giving anything but the basics.)
There was one job application I went through (and think it may have been a railway one) where they were happy to accept something from the tax-person that confirmed my employment history, rather than needing something from my (then) employer - you can (or could) get this sort of thing if you have a 'government gateway' account for income tax. but again this will vary by employer and maybe by role.