Changing the inclination manually is the right way. Consider that the terrains inclination itself must be flat or better positive in direction to the railway track. A hillock on both sides of the track ensures this.
I think its more of a bug. With or without hillocks, you can easily built railway bridges, but the game simply doesn’t allow road bridges the same way. This need a fix.
Found it finally, drag a strech of road and press m to raise it until a bridge is formed. Its interesting to learn these tricks just like a photo editing program like GIMP.
The Hillocks only work with rail tracks
I wish the M/N trick worked for the railtracks as well