Riders at two of San Mateo's three Caltrain stations must board from the southbound platform only after the agency posted a service alert Wednesday, Aug. 5.

The disruption affects Hayward Park and Hillsdale stations. Northbound passengers heading toward San Francisco who would normally board from the northbound platform must cross to the southbound side to catch their train.

San Mateo station was not included in the alert.

What single-tracking means

Single-tracking forces trains traveling in both directions to share one track instead of each direction using its own. That means northbound and southbound trains take turns passing through the affected stretch, and one train may have to wait outside a station while another boards or departs.

The result: delays. Caltrain has not announced a cause or end time for the disruption.

What riders should know

The Caltrain Alerts account posted the notice Wednesday afternoon, stating that all trains will board on the southbound platform at both stations. Riders should look for signage and listen for announcements directing them to the correct platform.

Caltrain's safety guidance reminds passengers to stand behind the yellow line at all times, which is especially important when trains from both directions are using the same platform.

The two affected stations sit about 0.9 miles apart on the Peninsula corridor in Zone 2. Hayward Park is near Highway 92 and Hillsdale is near Hillsdale Shopping Center.

How to stay updated

Riders can check live service alerts and a real-time train map at caltrain.com/alerts. Text and email notifications for specific train numbers are available by subscribing at caltrain.com/text-alerts.

For questions or to report an issue, Caltrain's customer service line is 1-800-660-4287, available weekdays 7 a.m. to 8 p.m. and weekends and holidays 8 a.m. to 5 p.m. For emergencies, call 911.