Map of US highway 87 and family
(Note: the two maps below are intended to display side-by-side. If you have a smaller monitor, the second map may display underneath the first.)
The map on the left shows the original US 87 family - note that US 87 itself went only as far south as Rawlins WY, and that for most of its length it was split between east and west branches. Compare that to the map on the right, which shows US x87 routings after about 1940:
US 87 is represented by the purple line from Havre MT to Port Lavaca TX. It is no longer signed through Colorado (actually between Raton NM and the Wyoming line), so that segment is shown in red.
US 187 (historic) is the red line from Rock Springs to Moran Jct WY.
US 287 is the blue line from Choteau MT to Port Arthur TX. US routes are not signed through Yellowstone National Park, so I've indicated the implied routings with a dashed line.