Planned engineering works require many replacement buses in many instances, for example Three Bridges to Brighton/Lewes can require 90+ vehicles each day over a weekend!
Operators need to find suitable vehicles and still operate their own scheduled services, so lots of school transport is used.
There's a knock on effect!
School transport is generally provided by councils under contract, at the cheapest rate possible, vehicles used on schools journeys are not subject to dda compliance, and therefore are older step entrance vehicles bought cheaper than new vehicles for this work.
Now the Rail Replacement work. Once the dda compliant vehicles have been allocated to the 90+ workings, that may leave say 40 uncovered workings! Vehicles are then sourced to cover this work, again as cheaply as possible and in comes the school operator with the older step entrance buses and coaches. Which, like the bigger groups use vehicles used during the week on services that run Mon-Fri only.
Therefore the only time a rail replacement bus (service) could be guaranteed to be accessible is when all vehicles carrying pcv licences have to be accessible no matter what the use!
Saying that, if a journey required more than one vehicle, most sensible planners and supervisors would run one accessible and one non.
All this is for planned engineering work though, emergency response or E - rail is classed as the emergency evacuation of persons and anything with a wheel in each corner and a windscreen could (and often does) turn up. All quite legally!
As for the "Is it a service or not? "
Well, technically - it's a private hire, hired by the rail operator, otherwise there would have to be a service registration with the traffic commissioner for every rail replacement, detailing stops, likelihood of relief journey being required, listings in N&P, showing every operator.