My MDF spline.

A couple of photos showing how I did it:

This is part of a spline on my railroad that I pulled out to install a bridge.

The section shown here is about 1.2m (4ft) long.  The wide spacing on the ties were due to the fact it was eventually going to come out however it served well.

Next photo shows the construction:

Construction is 30mm x 30mm pine blocks at about 100mm (4") centres with 2 layers of 30mm high x 3mm MDF strips that I cut carefully using a handsaw.  Glue used was common PVA glue.  Subroadbed on top is 6mm balsawood which worked well for spiking handlaid track.  

Supports at about 450mm (18") centres though I pulled the supports under this section before installing the bridge and had no noticeable deflection over a 1.0m span with trains going over.

I needed a LOT of clamps but once you got it working you could get quite a bit up in an evening.

I used 1/4" MDF to make

I used 1/4" MDF to make spline roadbed on my second layout 13 years ago.  It worked really well.  Even better, the sellers agreed to rip the single 4X8 sheet I needed into a bunch of 15/16" strips while I watched.  BONUS!! It netted me about 45 strips eight feet long, and at six ply I got a nice folded loop main line out if it, plus enough for a couple of switchbacks.

MDF doesn't like track nails.  It takes wood glue well, and you'll need about eight small clamps for every three-ply lamination you do at a time.  The clamps don't just keep the plies together while the glue sets, but they also keep the plies from sliding along the curvature you design and fashion.

Tip: use the risers, not the splines, to make any super-elevation you desire.  You still place the spline plies flush along the tops of the risers, but you tilt the risers themselves a few degrees off vertical to achieve your super-elevation.  Don't overdo it, either.