Honolulu's transit tax collections rebounded 10 percent in May to $14.9 million compared with a year ago. However, the city will need to collect a net $41 million this month or face a shortfall in collections for this fiscal year.

The city needs to collect $188 million during the 12 months ending June 30 and a total of $4.1 billion by Dec. 31, 2022, according to financial plans. Those tax collections, which started in January 2007, combined with $1.4 billion in federal grants, will be used to build a 20-mile elevated commuter train from East Kapolei to Ala Moana.

Through the first 11 months of the current fiscal year, the half-percentage-point general excise tax surcharge raised a net $146.7 million for the city, based on figures provided by the state Department of Taxation. That's down nearly 4 percent, or $5.6 million from year-ago levels.

Overall, the tax has raised $364.2 million during the first 29 months. That figure, and all figures in this article, do not include the 10 percent the state takes off the top to pay for administering the tax.

In the fiscal year starting July 1, the city's financial forecast anticipates transit tax revenue growing to an inflation-adjusted $198 million, or $16.5 million a month.

City officials expect any near-term revenue shortfall will be offset by higher collections in the future.

Officials want to begin construction in December, contingent on federal approval, and launch service in phases between late 2013 and 2019.