It wouldn’t have had much of an effect. Blowing up a few bridges would have set the opening of the transcontinental railroad back a few months or years, and with it the development of the west in general, but it would not have completely stopped. There was already a stagecoach route, telegraph line, ships to Panama and Cape Horn, and the Oregon Trail (which kept being used years after the railroad was finished). But any damage caused by a few men would have quickly been repaired because it was a massive project backed by Wall Street and the Federal Government.
If the damage had been significant enough to make the line no longer viable, it might have taken a few more years to scout and build an alternate route (other transcontinental railroad projects were built in later years, so one of those would have been accelerated in this scenario).
So the settling of the west would have been delayed by a few years, but not stopped. This would probably have a bigger effect on states in the rocky mountains and the southwest since you could still get to California, Oregon, and Washington by ship. The worst that could happen is perhaps a few delayed statehoods.
