Railtrack has filed a £20m claim against Thames Trains blaming their driver for last year's Paddington Rail crash in which 31 people died and 227 were injured.
The notice has been received by Thames Trains' insurer St Paul, which also insured the other train company involved in the accident, Great Western Trains.
A statement issued by St Paul said: "The Railtrack claim carried allegations that the Thames Train driver was to blame for the incident."
The insurer and Railtrack made clear that the claim in no way seeks to pre-judge the official inquiry into the Paddington rail crash under Lord Cullen which begins next month.
The crash is estimated to have resulted in £20m worth of damage.
A Railtrack spokesman said: "We have lodged a claim against Thames Trains in accordance with official regulations. This requires parties to file notices of any claims within 90 days of a rail accident."
Railtrack had issued the claim to protect its insurance position, the spokesman added. It declined to reveal the identity of its own insurer.
Meanwhile, Railtrack said it had received counter-claims from both Great Western and Thames Trains.
Critics of rail privatisation say the complex situation is the result of British Rail being split into more than 100 operating companies.