Moffat Tunnel Question

On my recent Amtrak CZ trip we detoured, from Denver to Salt Lake City on the UP Overland route because of work on the Moffat Tunnel.
The Amtrak literature on the detour stated that it was work being done by the Union Pacific. In my dumb guy world, I had always associated the Moffat Tunnel with the CB&Q, D&RGW and WP and wasn't aware of how the UP got in the story.
Help!

D&RGW, which evolved into a well-managed and profitable road during the 1960's, was later acquired by Denver oil magnate and venture capitalist Philip Anschutz, who used its solid standing to leverage control of much-larger Southern Pacific. When SP was later merged into Union Pacific, the component roads of the CB&Q-D&RGW-WP route used by the long-gone California Zephyr found themselves in opposing camps.

The western transcontinental market is down to two carriers, BNSF and UP, and both have main lines with milder grades, higher capacity and better signalling than the former Rio Grande, which now handles only the Zephyr and locally-originated traffic, primarily coal.