Do they not already have this responsibility to a certain degree under the Occupiers Liability Act 1996?
Given that the access route is to their works. I would suspect it would come under their risk assesment planning for the water company and if not , certainly the haulage company should have prepared one for safe use of the crossing just as they should for any low bridges on the route
Lack of understanding of English or UK railway crossing regulations is no excuse. It is no different to a tipper lorry tipping under over head power lines on private land and causing an accident. Both he haulier and landowner are liable