Step-by-step screenshots illustrating junction design.
You can to moderate speed junctions in Railroad Corporation only if you connected to the OUTSIDE of a moderate speed curve.
With the current game mechanics proper junctions between double-tracks are not yet feasible. So I'm using cities/stations as the connecting points.
1. Start with 60mph curve.
2. Mirror it.
3. 120mph possible at inside of Y.
4. Now do the other side.
5. Upgrading the station.
6. Full-size station doubling outside lines.
7. Other one. Almost 60mph : can be done smoother than this.
This can be done on both sides of the station, allowing each city to be connected to 6 others.
The principles of outside connections can be adapted for many situations to make moderate speed junctions on single tracks away from stations. But in my opinion at present these are only reliable at low volume.
Extra: Short Sections, Credit to Coenvijge
Also, don't forget to place more track markers to break your track up into smaller sections which will allow trains to follow more closely. His analysis on section length.
Straight bridges and tunnels can be done in small pieces: lay one piece of track and without moving the cursor click again to lay 21 miles of perfectly straight, flat track. Keep clicking: each click adds a new piece.
When climbing hills, longer sections will give lower grades. These can then be manually broken into smaller pieces. In Construction Mode the "telegraph poles" represent section boundaries.
1. Start point.
2. Second point must be in-line.
3. Signals indicate boundary to select for scrapping.
4. The remaining "telegraph poles" denote the new "section".