Dodgy fishplates? Try bridging the rail joints with a wire whilst the loco is on the track and see if you get a speed variation..
Are you using the point blades as electrical switches? They can be a bit iffy - try bending up the little bits that wipe on the bottom of the moving blades to make the connection better..
It was mentioned - http://irishrailwaymodeller.com/showthread.php/2592-doing-ground-cover?p=39353&viewfull=1#post39353 - but we were being gentlemen and assuming that you were keeping quiet about it...
If you use this to look at the older maps, then you can often find reasonable views - http://maps.osi.ie/publicviewer/#V1,588882,739883,0,10 - it just depends what was there at the time and what records are available now..
I think that it's very good of you to have set up such an excellent regular exercise programme for her. She's lucky to have such a caring man and I hope that she is suitably grateful for her good fortune.