In the "Old World" you can upgrade your citizens from "Farmer" to "Worker" than "Artisan" and than to "Engineer". For this achievement you need an island with a river on it.
Dirt road is the 1st option in construction menu under the "Farmer" tab, start building a dirt road that crosses a river, if you have 20 timber there will be you 1st bridge, the wood one.
Paved road is the 1st option in the contruction menu under the "Worker" tab, start building a paved road that crosses a rived, if you have 20 bricks for the bridge and at least 1 more for the road on both side of the brigde, than you second bridge done, the stone one.
Railway is in the 2nd option in the construction menu under "Engineer" tab. You choose "Electricity" and there is the railway. Start building railway that crosses a river and if you have at least 42 steel and 2 timber for the bridge and 1-1 unit of railway for have a railway on both end of the bridge, than there is your 3rd bridge, the railway one.
Same as console edition.
https://www.trueachievements.com/a386680/isambards-kingdom-a...