Passenger carrying non-CDL stock is legally prohibited on railways with speeds >25mph by regulation 5 of the Railway Safety Regulations 1999. However regulation 6 allows the regulator (ORR) to issue exemption certificates which will specify the minimum number of stewards. This is usually (if not always) effectively 1 per coach, though this can be distributed as one steward supervising 2 adjacent vestibules of 2 doors each at the coupling between 2 coaches.
With CDL fitted, there is no legal requirement for stewards, however there is a requirement (though I can’t find the reference) to ensure that human appendages remain inside the vehicle. The operator must have a risk assessment for passenger operation of hinged doors which would likely examine the risk of opening windows and trapped fingers, mitigated by steward operation of doors, with the same numbers as non-CDL.
There is a legal requirement (PRM-TSI on the main line, ROGS elsewhere) for accessibility, which for hinged door stock will also drive a need for stewards.
A few railtour operations run with volunteer stewards, so the cost is effectively nil. (Don’t confuse cost for value!!)
