This is a commonly used item for modellers when laying track, I have created this item in a two and three-track version.
The three-track version is useful when laying track in fiddle yards and where you have more than two tracks running parallel.
The distance from the centre of the track to the next track centre is 50mm.
I have tested this design on a Bambu Labs A1 FDM printer and printed it inverted.
When printing on a FDM printer I recommend you use a filament like PETG or PLA+. you can use PLA but be aware of the limitations of PLA.
The examples in the pictures were printed on a Bambu Labs A1 with a 0.4mm nozzle and a layer height of 0.12mm using PETG.
Additional Info
Please check my website, https://sites.google.com/view/pete3dprintinghelp/home
My website has lots of useful info.