WHP is a towered airport, so it would definitely have been open and thus a "controlled airport" at 2:00 in the afternoon. Even if the officers hadn't called it in, the tower would likely have an easy way to contact Metrolink, seeing as the rail line runs just outside the fence. As for which ones might be a factor, they would know it's the Antelope Valley line simply because of the location of the tracks. And they know where each train is at all times. Since it was on a weekend, that line only has six trains going in either direction, and the trains are pretty evenly spaced throughout the day. So they would know exactly which train would be an issue, since there was probably only one in the area. Commuter trains are also able to come to an emergency stop relatively quickly. For comparison, a fully loaded 10-car E956 "ALFA-X" (the newest train in the JR East inventory) is designed to come to a dead stop from 225mph in 13,000 feet (2.46 miles). Metrolink uses 4-car trains from Hyundai that top out at about 80mph, so they would be able to stop significantly faster. In fact, Metrolink trains are supposed to be able to stop in no more than a mile, a distance easily visible on a straight-line section of track. But an emergency stop wouldn't have been required if they had been informed.