I have recently laid some track with concrete sleepers using materials I purchased a couple of years ago from C&L Finescale in the UK.
Although intended for 4mm OO scale modelling, the size and spacing of the C&L sleepers are pretty close to Sn3.5 scale. The sleeper spacing (pitch) is 10 mm and, although I normally space timber sleepers at 12 mm, the closer distance doesn't look wrong.
I purchased ready made track which included Peco Code 75 rail. As I prefer weathered rail, I replaced the rail with Micro Engineering Code 70 rail. Code 83 would also fit as the base of the rail is the same as Code 70.
C&L also sell the concrete sleepers on their own, which would be the way to go if you want the look of weathered rail.
So far, I have completed just under 2 metres of track with the concrete sleepers. The first section was time consuming, as it is on a very tight curve (600 mm radius) and I wanted to make sure the curvature was even to avoid any tight spots. For this section, I glued the sleepers, which come joined in groups of 8, to the MDF roadbed using PVA glue. I 3D-printed a guide strip which fitted inside the simulated Pandrol clips, to maintain even curvature. Even though I pre-bent the rail to the required radius, I was quite difficult to slide the rail into the sleepers and I found the maximum workable length was about 20 sleepers at a time. So, I had to cut the rail into shorter lengths for this bit.
Once I got into the straight track, it became much easier. One technique which worked was to slide both rails into one group of 8 sleepers, then clamp the completed end of the track section onto my workbench with a scrap of plywood on top of the rails. Then, I could easily slide the remaining groups of sleepers onto the 2 rails.
At this stage, the straight track isn't glued down, but I intended to apply dabs of PVA glue every few sleepers to hold it in position until ballasting.
It is essential remove any burs from the rail ends before assembling the track. Also, it is necessary to cut the small joining pieces between the sleepers on one side for curved track. Unlike normal flexible track, there are no gaps in the sleeper moulding.
One slight annoyance is the tops of the sleepers have raised lettering moulded in which says "COSTAINS", which is a UK manufacturer of real concrete sleepers. As my layout is close to eye level, the lettering is not obvious, but it could be filed or sanded off if necessary.
C&L Finescale also make timber sleepered track with suitable sleeper size and spacing. However, it is only available with bullhead rail (not flatbottom) so it is not usable for WA modelling.
