Tunnel and overhead wiring inspection 

That's quite possibly what has happened, like you say the easiest fix will almost certainly be to dig out the floor and lower the track a bit. Part of the fun of garden railways is all the interesting real world engineering problems.

They're mostly native plants or deliberately planted garden plants, not weeds!

I think of plants not as weeds because they can make the place look more natural and more realistic removing plants destroys this look entirely.

I don't use any sort of lubricant or similar on the wire normally. I've experimented with graphite trolley pole heads though which seem to work really well, not just for electrical conductivity but also the lubrication means the poles track through all the overhead wire frogs better. I haven't yet found a fully practical design though, the experimental version used a little bit of pencil graphite. Otherwise the trolley poles are either steel or bronze, this tram has fixed (sliding) bronze trolley pole heads. Other trams have brass wheels, which operate smoothly but aren't as reliable for electrical contact.

Not often, probably only a couple of times a year when there has been a kangaroo or rabbit or something trip on it. The wooden poles last 3-5 years so they need to be replaced from time to time, but I just do it as required.

sure does!

I have some battery powered units where the real thing is either steam or diesel, but the reality is the overhead infrastructure takes a lot less maintenance than batteries do. You have to charge batteries and keep track of the charge state whereas electric trams are always ready to go. It would be a lot more expensive too.

This is 1:24 scale and 45mm gauge, so a bit bigger. Would be cool though!

That part is called the trolley pole. It has a small U shaped part on the end that sits on the wire.

It's much easier to see the wires in the dark, during the day the sun shining on the camera would be too bright.

Not in winter.