When I called in at local plant depot railway road vehicles where topping up with red diesel. Asked storeman about it and was told they had to record exact amounts so used and then were charged the appropriate tax difference between white and red.
Red diesel can only be used in agricultural and plant vehicles (like cranes, tar sprayers and road rollers) when on the road. Agricultural vehicles are limited to their road mileage before their exemption is nullified. NO other vehicles can, so the cars/vans were being used illegally and there is no such thing as payin gthe extra tax - it is simply illegal!!
Company cars, vans, lorries etc would be equal in being used illegally. Red diesel must not be used in vehicles used on the public highway. outside the very few exemptions. What you observed was excise evasion and a very serious criminal offence.
PS I worked on resilience arrangements for the emergency services during the fuel strikes of the early 2000s (farmers revolt) and there was no provision for the use of red diesel by the Police, Fire or Ambulance services - so I cannot believe the railway companies had approval. All we had was access to the limited fuel supplies at petrol stations, as did teachers and NHS staff. This was on production of ID cards to ensure public services were maintained by staff being able to get to work. Emergency service vehicles were able to use the same and the blue light services entered into agreements to pool fuel resources and cease any non-emergency driving. Red diesel? NO!!