I agree with Gator,
On my BN/BNSF DRGW/SP I went from photos as to where to place Ditch Lights (DL) on the models as most were installed above the walkway on the prototypes. As far as an FRA mandate to the location on a given unit, probably there was a now lower than** no higher than** type thing. I have seen DL's above the walkway on one end and below on the other of the same locomotive. I remember it was 1997ish that lead locomotives had them on a regular basis on the head end. You can still operate a locomotive on the point w/o DL's, you just can not go over a public xing exceeding 20 mph. It is a real pain too, if there are a lot of public xings. Perhaps a reason they may have been mounted above the walkway was to try and keep them from being broken out hitting stuff, or maybe even because of ease of maintenance or installation at the shops.
On my Modern freelance QA&P I did like Gator said, put them below the walkways because it is sooo much easier.
Hope this helps.