Obviously they weren’t supposed to. They were traveling through a freight yard so speed should have been at slow yard limit unless they were on a faster bypass track. So the cause is one of four reasons — the engineer was speeding and could not stop in time, he ran a red signal, a signal malfunctioned, or the dispatcher routed him to a track already occupied. We will know soon enough as the modern engines have data recorders.
I went to CNN and found this
Hostages held at Clinton ‘08 office in New Hampshire