Inertia. A multiple ton train cannot brake on a dime.
Trains can't stop quickly or swerve. The average freight train is about 1 to 1¼ miles in length (90 to 120 rail cars). When it's moving at 55 miles an hour, it can take a mile or more to stop after the locomotive engineer fully applies the emergency brake. An 8-car passenger train moving at 80 miles an hour needs about a mile to stop. http://www.minnesotasafetycouncil.org/OL/stop.cfm
So the train has to travel about 1 entire train length or more to stop, without causing a catastrophic derailing. Considering the speeds given, and the non linear speed changes, it will take a few minutes to get to a full stop, and may not be immediately detected visually. They would be slowing down, you just can't tell yet.