INITIAL REPORTS from the National Transportation Safety Board (NTSB), based on data from sensors and interviews from the train crew, suggest that the train's emergency brakes signalled the start of the incident.
There has been speculation that a fault with a tanker or wagon could have been the trigger for the blaze.
It is still unclear who will be held responsible for the fire. CSX operates the tunnel, track and locomotives, but wagons and tankers are owned by individual shipping companies.
As a result, responsibility for rail-worthiness could fall to them.
Train, track and tunnel owner CSX was criticised in an independent audit report last year for having substandard track across its entire operation.
But CSX spokesman Robert Gould said that since the report was published the tunnel had been refurbished.
'The tunnel was inspected in April 2000. There were problems, mostly to do with drainage, and as a result the track was completely relaid. Before this week's accident you were looking at a completely new railroad.
'At this stage CSX is taking no responsibility (for the fire), ' Gould said.
'We will wait for the NSTB to report, and until then it is not our place to comment.'
In the meantime, the cost will have to be borne by the State of Maryland.
'The Department of Public Works will bear the costs of repair to the roads and utilities, ' confirmed DPW director George Winfield. 'Then it is a matter for the lawyers.'
Damage and disruption caused by the fire could run to tens of millions of pounds. First estimates from the DPW place the cost of repairs to Howard Street, which was damaged by heat and water from the ruptured main, at £350,000.
'This is not including the main, this is not including light rail and this is not including claims from the utilities, ' said Winfield.
Winfield expects the total bill including utilities repairs to exceed £700,000, aside from any claims from local businesses affected by loss of trade.