It's a good idea, and it shouldn't change how mountains are used as a defensive terrain if you can only build it in your border, and it takes 2,3 times as long.
Also, I think we should have highways replacing railroads. You could reduce micromanagement by constructing a building in a city with a modern tech that requires 1 oil (to represent asphalt maintenance and to prevent it from being spammed). This will turn the present railroad connection from the capital into highways, so you don't have to use the workers for the third time again.
The advantage of highway over railroad could be that your own population moves between cities connected by highways in World-War style invasion, or even stimulating urbanization by shifting population for your non-productive size 8 cities into your productive pop 25 ones, going from 25 to 30 would be more valuable than going from 8 to 3 would hurt. or reduce the bonus of railroad a bit and add the rest to the highway. I don't think highway should be faster though.
The disadvantage would be higher maintenance and oil requirement, so not all cities and towns would get it, some remote towns in Canada is only connected by rail, and it seems approporiate for mining towns. Also, if the population did shift between cities connected by highways and not rail, that would be valuable for remote outpost towns that is a mining town for strategic resources that you want to keep population at a certain level.
Edit: Also, I just had an idea, if you apply my highway idea to the railroads and make railroads appear immediately in cities that's already connected through the road if you construct a building that requires +1 iron (for the railroad tracks), you can save the repetitive worker micromanagement and the uselessness of iron in the industrial age at the same time!
This would also make Russia the world leader in railroad construction, that's be supremely entertaining, maintaining a huge trans-Siberian empire while other countries have to pick and choose most important cities.
Someone from the studio, please read this.