Central Trains and ScotRail — both of which are owned by National Express — have been criticised by RDS for routinely providing buses as a replacement for trains.
Executive officer Nat Taplin said: "Road coaches should not be used to substitute or supplement trains on a routine basis. Where there is increased demand, operators should lease extra rolling stock. Rail passengers pay for a seat on a train."
In a letter to Rail Regulator John Swift in October, Nat pointed out: "There is no shortage of rolling stock. There are 700 spare units in storage around the UK. A consignment was even sold to New Zealand last week.
"We urge you to obtain a clear undertaking from National Express that road coaches will not be routinely used as an alternative to leasing adequate rolling stock."
He pointed out that there had been problems of overcrowding and bus substitution between Shrewsbury, Aberystwyth and Pwhelli, Perth and Inverness, Nottingham and Skegness and on local services in the West Midlands.
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