Does CBP have access to domestic Amtrak reservations?

Documents released to us by Amtrak suggest that since 2012, US Customs and Border Protection (CBP) has had direct access to Amtrak’s reservation system, possibly including access to reservations for Amtrak passengers traveling entirely within the USA.
What do these documents show? And why would an immigration and border patrol agency want access to records of travel by US citizens and other residents within the borders of the US?
The Amtrak documents we received this week are the fourth in a continuing series of long-overdue interim responses (1st interim response, 2nd interim response, 3rd interim response) to a FOIA request we made in October 2014 for records related to Amtrak’s data-sharing and other collaboration with the Department of Homeland Security (DHS) and other US and foreign law enforcement agencies.
Most of the latest tranche of Amtrak documents we received are copies of e-mail messages within Amtrak and between Amtrak and CBP in the immediate aftermath of Amtrak’s systemwide transition from paper tickets to e-tickets on July 30, 2012. Those messages suggest that Amtrak’s transition to e-ticketing was followed by the addition of ‘pull’ capability for CBP to retrieve Amtrak reservations to the already existing ‘push’ system of Amtrak sending passenger lists for cross-border trains to CBP by email.

This post was published at Papers Please on Sept 23, 2015.