Give your eight year old kid a handsaw and have him cut it.

My dad made car ramps from sleepers, I cut more than half of the bit’s of sleepers
as he was doing the measuring and nailing together. We used an old hand saw.

[strong troll] You could always ask a man to do it for you[/strongtroll]

Having worked on a big garden project recently that was lined with (new) sleepers, chainsaw is the way to go. We cut through them in seconds With ease

Recently cut 32 green oak sleepers with electric chainsaw (same one as the screwfix link above). Worked a treat.

I installed around 40 large softwood sleepers, I tried a couple of methods but by far the fastest was an electric chainsaw. Probably 100+ cuts.

I didn’t need to worry too much about the cuts so did them all freehand, and used a basic chain which I suspect is a ripping chain.  As I was freehand, the chain would skate around slightly on the surface before digging in.

If you get a suitable chain, and make some sort of jig or guide for the chainsaw you could probably make a very good job of it.

Having watched a boatbuilding series on YouTube, I’m sure you’ll find there are various modifications to create a fixed chop saw style chainsaw, also I’ve seen a second handle bolted to the end of the bar so you can have someone both ends.

Hand cutting was incredibly slow, and my sharp circular saw just started smoking and burning, probably as the wood was damp.

I bet you can rent some industrial bit of kit form a hire place that would to the trick. Some form of portable band saw I would have thought would be best.