[Aside: this was a mail message I sent to <someone> about the railway
tunnels in Glen Farg.  '>' was my original message, and non-'>' a followup
I wrote slightly later with more information.]

[...]  You want Landranger sheet 58 
(Perth & Kinross) or (better) Pathfinder sheet 361 (Bridge of Earn & 
Dunning).

> I walked the tunnels before the M90 was built.  In fact, thinking back it
> must have been around 1971 or 1972 or so, because the multi-storey bridge
> was obliterated by the M90 section from Glenfarg to Milnathort, and I think
> that was put into use around the time I was learning to drive (1973).  So I
> was able to pick up the old track-bed at Paris Bridge (the said three-level
> one), but that's not possible any more.

Paris Bridge is at 137114.

> What you would have to do now is go a little way further down the Glen 
> itself (I'll look up the grid references tomorrow).  There's a layby on 
> your left at the foot of a farm road, and you can pick up the railway on 
> your right just across the B somethingorother, formerly the A90 (while 
> you're there imagine what it was like latterly with convoys of huge trucks 
> heading both ways through the Glen!).

Layby at 149126.  The road is the B996.

> Heading North, it's only a short way to the viaduct, with the first of the 
> tunnels (the straight one) just at the other side.  It was in reasonable 
> condition 30 years ago, but I don't suppose it's been maintained since.  
> You come out high above the Glen, just to the North of the Beinn Inn.  The 
> second tunnel, with the bend in it, is just a little way further on.
> 
> The bend is there, incidentally, to align the track along the North face 
> of the Ochils.  You'll come to the remains of the bridge over the Wicks 
> O'Bagilie road not far out of this end of the tunnel.  The railway used 
> to descend gradually into the Earn valley, taking a broad sweep round 
> into Bridge of Earn.  I'm not sure how much of the embankment remains; the 
> M90 splatted lots of it.  There used to be a junction at Bridge of Earn, 
> with a station, but all that's there now is the single-track line to 
> Ladybank.
> 
> Getting back to the layby: you could either retrace your steps, or you can 
> head up the hill on the minor road.  Eventually you'll come to a farm road 
> (I'll have to check the map on this one), which will take you back down to 
> the layby in the Glen.  Last time I walked this section was along the M90 
> itself before it was opened to traffic.

If you come back by the minor road you'll want to take the farm road to 
Meikle Fildie at 143134 or thereabouts.  It should wind its way down to 
the Glen at the layby.

> For a longer walk through the hills, you could take the Wallace Road from 
> Lochelbank farm (I think) over the hills, and then come back along the 
> railway.  I'll certainly need to check the map about that one.

The car park is at 135126 or thereabouts.  IIRC the Wallace Road is now 
signposted.  If you go this way around you'll want to aim for the remains 
of the railway bridge over the Wicks O'Baiglie minor road at 153150.  
You'll have to come up the Meikle Fildie farm road from the layby on the 
B996 that I suggested as a parking place above.