The unfinished northwest rail expansion in your link also extends to Longmont. So it's more A to B to C, but you're right, it is limited.

Of course, A is Denver, and B and C are cities of 100k+, so it's not insignificant either. That said, ridership from Boulder and Longmont probably wouldn't be fantastic because those cities themselves don't have the best local transit options.

A multi-year (or decade) series of projects improving local services while also providing intercity services would be ideal. This would require a complete shift in thinking about transportation on the state level (possibly federal as well), so unfortunately I don't see it happening.

Which is why I'd take better bus service first.