The head of Network Rail has said he does not know why half a specialist team working on the West Coast Mainline over New Year did not turn up for work.
Engineering work at Rugby caused delays for thousands
Ian Coucher told a House of Commons inquiry attempts to recruit others at short notice had proved "fruitless".
A shortage of overhead linesmen meant engineering work between Rugby and Northampton, which should have finished by 30 December, overran by four days.
Thousands of passengers had their journeys disrupted.
There was more disruption in London when late-running work at Liverpool Street station forced it to close as people were returning to work.
No penalty clause
Network Rail could face a fine from rail regulators as well as having to pay up to £10m compensation to passengers.
The Commons transport committee heard rail specialists Jarvis had supplied a list of specialist overhead linesmen it would be using for the work at Rugby.
In the event, many did not turn up - up to half the 70-strong team were not there at one point.
The MPs were told that so far, Jarvis had not provided a "satisfactory justification" for this.
The MPs also heard there was no penalty clause in Jarvis' contract.
Mr Coucher, chief executive, said: "They will get paid for some work, but not all of it, but there's no penalty - we can't flow down the extra costs that we incur down to them," he said.
He said there was no penalty, because normally if the work is not carried out satisfactorily then the company does not get additional work.
He also said Network Rail had been overambitious in its New Year programme, but said the firm had been desperate to deliver its work in a way which did not require extra time.
Praising Network Rail staff he said: "When we called on Network Rail maintenance people they came and they worked and they were loyal and they worked so hard we almost had to sent them home for their own safety."