Less than six months ago, Caltrain was warning passengers that its weekend service could come to an end.

In a complete U-turn, the agency announced on Thursday that it would actually be expanding its weekend train runs.

Starting Jan. 1, Caltrain will begin operating its “Baby Bullet” trains — express lines that only stop at major stations — during the weekends as part of a three-month trial program. The trains will run concurrent with Caltrain’s normal weekend operating schedule.

The Baby Bullet trains will make one round-trip in the morning, and one round-trip in the evening, accounting for four trains in total, according to Caltrain spokeswoman Christine Dunn. To keep costs low, one crew will work all four trains.

Earlier this year, Caltrain approved a set of service reductions and fare increases to overcome a $2.3 million budget deficit. Also part of that package was a commitment to pursue revenue-generating possibilities, which included weekend express trains, a suggestion favored by many Caltrain riders.

After the three-month pilot period has been completed, Caltrain officials will review ridership and financial data from the program to see if it should be extended further. For now, the Baby Bullet will run between San Francisco and San Jose and stop at seven stations — Sunnyvale, Mountain View, Palo Alto, Redwood City, Hillsdale, San Mateo and Millbrae.

wreisman@sfexaminer.com