It has emerged more than £2.1m has been spent on the inquiry, which is being chaired by retired judge Lord Hardie.
The investigation was launched to establish why the project was delayed, cost more than originally budgeted and delivered significantly less than expected.
Despite getting underway in June 2014, the inquiry has still to hear from its first witnesses, with investigators currently considering at least six million documents relating to the scheme.
It has also emerged that the sheer scale of the probe, and the fact the Scottish Government goes into lock-down because of this year’s Holyrood election, means it will be Spring before anyone takes the stand.
Sarah Boyack, Lothians Labour MSP, said: “I’m sure the public be as bemused as I am at the rising costs of this inquiry.”
A spokeswoman for the Edinburgh Tram Inquiry said: “All efforts are being made to ensure it delivers best value for the public purse.”