Remembering that the railways fall under different legislation (a lot) from the UK Domestic Driving and EU Driving Hours regulations, are we not talking here about two different subjects?
Harking back to the original post, if your job (as a PSV Driver) includes driving buses on a regular basis, then you will not be able to work, regularly, rail replacement shifts at a weekend.
Most rail replacement shifts fall under the EU legislation (over 50kms), and as such any service driver in the UK would then fall under these regs. Their employer then, by law, needs to keep track of them under EU regulations, which causes a lot of problems with domestic duties and breaks - not to mention rest periods per week.
For example, on a domestic rota, you may have two rest days per week, but they may be split (Monday, Wednesday say). On EU, you require 45hours unbroken rest per week, or if not, this to be compensated by the end of the third week.
You'll find bus operators keeping EU and Domestic work seperate, and the current crop of split registrations allows for most work to be Domestically rostered.
There are exceptions (as with anything!), certain rail replacement services (shorter ones) are registered and operated domestically, although this is rare, making the 6 day working week described by notadriver possible in those circumstances.
Hope that helps.
Cheers
