What is the policy on bus replacements services if the bus's are not wheelchair user accessable.
These days, all rail replacement services should offer wheelchair accessibility. I believe that where multiple vehicles are used on the same service (at the same time from the same place), the second and subsequent vehicle may not need to be wheelchair accessible, and wheelchair access may not be to the whole of the first vehicle either. Rather like a train - 5 carriages but only one end of one of the carriages needs to have wheelchair facilities.
Did you have a specific situation in mind in asking the question?