The previous version was named Amtrak-System-Map-020923.pdf and it's still available at https://www.amtrak.com/content/dam/projects/dotcom/english/p... and it really is about 1MB. Just zooming in, it seems to be a single lossy image.

The current one is called Amtrak-System-Map-081325.pdf and went live on the site sometime between August 19th and 20th (according to Wayback Machine snapshots). It looks like someone uploaded a working copy and forgot to export a nice small version.

It's somewhat akin to how documentation inside code is so often inaccurate; it may have been fine when it was written, but it doesn't get updated.