FirstGroup, which has been awarded the West Coast rail contract, says it still expects the transition to go "smoothly" despite legal challenges.
The bus and train operator beat Virgin Trains - which has run the service for 15 years - in a bidding contest over the London to Scotland line.
But Virgin questioned its rival's numbers, and launched a legal challenge against the Department for Transport's (DfT) decision to award to deal to FirstGroup.
As a result, the Government has not yet signed off the contract for the new 14-year West Coast mainline rail franchise.
However, in a trading update FirstGroup said it was focussed on ensuring a "smooth transition" for staff and customers.
"We have every confidence in the DfT's process which is rigorous, detailed and fair and in which bids are thoroughly tested," it said.
"We continue to prepare for a successful mobilisation on 9 December 2012."
The company, which already operates a number of rail routes including Great Western and ScotRail, is believed to have outbid Virgin's £4.8bn offer by bidding as much as £5.5bn to operate the line.
It said that performance across the rest of its business - which includes school buses and Greyhound coaches in the US - was in line with expectations in the first six months of this year.