Pretty much. It is obviously a for-profit freight system - In areas where the RR's top-dollar freight customers (especially domestic parcel delivery companies) want speed, they'll happily spend big to make that happen. And in areas where the RoI on speed (whether upgrades, or ongoing maintenance of existing track) ain't there, they can be happy with 25MPH maximums:

https://en.wikipedia.org/wiki/Rail_speed_limits_in_the_Unite...