Step 1: Place some tracks
Step 2: Build a tunnel entrance
1. Select the tunnel tool from the railroad toolbar
2. Press the CTRL-Key and click on the slope tile to build the tunnel entrance (without the CTRL-Key a tunnel will be build)
Step 3: Switch to underground mode
Press the "U"-Key to switch to the underground mode
Step 4: Build subway tracks
You can use the tunnel tool to build railroad tracks in a tunnel.
You can leave the underground mode by pressing the "U"-Key again.
How to build an underground station with bus stop
1. Build the underground station and write down the screen coordinates of a station tile (see the bottom right corner in the image)
3. Leave the underground mode. Move the cursor to the screen coordinates of the underground station (Important)
2. Build the bus stop