Responding to rail leaders’ update that the project might cost even more and won’t stay on schedule, Honolulu Mayor Kirk Caldwell is asking what did they know and when did they know it.

In a letter dated Wednesday, Caldwell asks Honolulu Authority for Rapid Transportation Board Chairman Don Horner and Executive Director Dan Grabauskas to explain what exactly they’ve learned that’s new since May, when this year’s Legislative session ended.

During that session, in which state lawmakers approved a five-year rail tax extension, HART officials provided those lawmakers with “detailed construction budget, schedule and procurement” estimates,” Caldwell states.

Now, about four months later, those estimates are different, he points out.

Caldwell asks Horner and Grabauskas to explain what they’ve learned since the session ended that prompted them to warn that rail could cost an additional $200 million to build, pushing the project’s budget shortfall to more than $1 billion. He also wants to know what’s changed to have them warn it likely will take at least a year longer to complete, pushing the completion date to sometime in 2021.

Horner and Grabauskas last week sent a letter with those updates to Caldwell and City Council Chairman Ernie Martin, two of the city’s top political players and political rivals.

The two top rail officials said that the additional $200 million is based on more conservative budget estimates. The delay is based on numerous factors, including court and contract bid challenges, changes in how HART will package the remaining construction work, and constraints to building the project so that workers don’t further hurt nearby businesses or increase traffic.

Still, Caldwell wants to know what’s changed since the session ended in May. His letter asks them to do everything they can to hold to the previous cost and schedule estimates, and to hold public workshops on the budget.