On Mon, 21 Jan 2013 16:09:47 -0000, shazzbat wrote:

Railway sleepers, or any type of wood, would be very slippery when wet,
...

Agreed, and real ex railway sleepers are filthy things full of tar, oil,
grease (and shit from the train loos). This muck will get onto your shoes
and most likely into the house...

... and would also rot away.

Real hardwood railway sleepers will take a very long time to rot (think
decades), not only does the oil, grease etc protect 'em they are pressure
treated with preservative. They are also big heavy lumps of timber, 10 x
5" and 8'6" long (commonly) weighing from around 1 cwt for softwood to 2
cwt for the denser hardwoods (50 to 100 kg).

The vertical (riser) board back filled with hardcore/gravel is probably
the best bet. The hard bit will be getting the rise and tread for each
step even within a fraction of a inch, people are very sensitive to
uneven steps and they can be trip hazard particulary if the rise varies.

--
Cheers
Dave.