The expiration of a federal grant will halt a popular suburban commuter bus service at year’s end, but transit advocates say it may be just a temporary stoppage.

Launched a few years ago, the IndyGo Commuter Express routes from Fishers and Carmel to downtown Indianapolis get 80 percent of their funding from a Congestion Mitigation Air Quality grant that will be depleted by the end of the year. The service has annual expenses of just over $1 million.

Other funding comes from $3-per-trip fares, and ridership has been strong enough to pay the local match to the federal grant solely through those fares, without the municipalities chipping in. During the course of operation, the routes have provided about 445,000 trips.

But without additional subsidy, the cost of providing that service would be placed on riders, who would face fares of between $7 and $9.

IndyGo and the Central Indiana Regional Transit Authority are optimistic they can find other funding sources. CIRTA is looking at other grants, among them federal funding that helps pay for so-called reverse commuting in which workers leave the city for work in the suburbs, but that also would require cash-tight municipalities to contribute.

“All indications point to a likelihood that a new federal grant will be matched with local public and private dollars, and a new commuter express service will evolve in the near future,” Ehren Bingaman, executive director for CIRTA, said in a news release.

He said the group hoped to have details confirmed and would provide more information by the end of the year.