In fact, Sarah is so fixated with tunnels that often she’ll make us drive through the Parkes Way tunnel, even if it’s in completely the opposite direction to where we are going. We once drove from Belconnen to Gungahlin via Parkes Way. See what I mean?

While I’m hoping it’s something she’ll grow out of, in the meantime, rather than wasting petrol on repeat trips through the Parkes Way tunnel (the walls are filthy and half the lights don’t work anyway), I thought it was time to take her on an adventure to a real tunnel – one she can walk through. So last Sunday, I grabbed our backpacks and walking boots and bundled Sarah in the Yowie Mobile and drove up the Federal and Hume highways to one of my favourite kid-friendly bush tracks – the Box Vale Walking Track.

This relatively level track follows the route of an old railway line which in 1888 was hacked by hand out of the sandstone to haul coal from the top of the Nattai Gorge, six kilometres through to the main Southern Line at Mittagong. Most importantly it features a tunnel! At its peak 100 tonnes a week were lugged out of the gorge and transported along the line. After mining abruptly stopped in 1896 the railway track, including its sleepers, were removed and in the 1980s the Nowra Lands Office designed and constructed the historical walking track which leads through the old railway tunnel.

Just five minutes into the three-hour walk and Sarah gasps, ‘‘Look Daddy, a tunnel,’’ as she crawls beneath a number of fallen branches lying across the sunken track. ‘‘That’s no tunnel, wait for the real thing,’’ I respond, trying to dampen her fever-pitched enthusiasm.

Apart from the dozens of branches and gums fallen across the track (no doubt victims of a recent wind storm), the track is punctuated by a series of fascinating cuttings. Natural regeneration in each of the cuttings varies considerably according to the soil, aspect and drainage. Just beyond the first of these – the aptly-named Casuarina cutting which is dominated by dozens of the strangely straggly trees – is a fire trail where it’s easy to miss the small triangular markers for the Box Vale Track. Some walkers make the mistake of turning right here and end up having to negotiate a steep 1.8 kilometre spur track which eventually leads to a waterfall – an adventure I’m saving for Sarah for a warmer day. Today, we’ve got a tunnel to find, so instead we cross the fire trail and continue on the Box Vale Track.

Although the railway sleepers were ripped up soon after the mine closed in 1896, parts of the timber trestle bridges that spanned the creeks remained in place for several decades until they were destroyed in the 1939 fires which ravaged the area. At Kells Creek, the circular holes that provided footings for the vertical timbers can still be seen in the creek bed.

While trying to guide Sarah across the rushing water without falling in one of these submerged holes, a couple of walkers approach from the opposite direction. ‘‘Look up there – there’s a koala!’’ they half whisper, half shout, not wanting to scare away their prized find. Not surprisingly the placid marsupial doesn’t budge, nor bat an eyelid (probably because like most koalas at midday, it’s half asleep).

What an unexpected treat. It’s the first koala Sarah has seen in the wild and the first I’ve seen outside of captivity this close to Canberra. Despite this rare encounter, Sarah has a one-track (no pun intended) mind – she waves farewell to the koala and pulls my hand insisting, ‘‘Daddy, we’ve got to get to the tunnel!’’

A kilometre or so ahead we descend into the last cutting before the tunnel. It’s like walking through a hidden rainforest. Surrounded by palms in a chasm only a few metres wide, it’s hard to believe trains laden with coal used to chug through here.

Sarah’s pace has picked up even faster now. If only she showed this pace getting dressed for pre-school of a morning I wouldn’t have to suffer the daily wrath of her teacher.

Finally, just beyond the fern cutting is the tunnel, and what a tunnel it is. This is no plastic drain pipe with a hunk of dirt chucked on top to make it feel like a tunnel. This is the real thing. ‘‘Let’s go in Daddy, Lets’ go in!’’ Sarah shrieks in delight. We’ve got no choice really for it’s the only way through this solid wall of sandstone. At 84 metres in length, the tunnel is short enough that you don’t need a torch but sufficiently long that you need to be careful not to trip over the corrugations where the sleepers used to lie.

‘‘Choo choo,’’ blurts out Sarah at irregular but voluminous intervals. Its fun playing imaginary trains, but the thought of being caught in here with a locomotive hauling coal heading towards you doesn’t bear thinking. Ten minutes walk beyond the tunnel we reach a cleared area where the train was loaded with coal from skips that were hauled up a 45 degree incline from the mine 160 metres below in the gorge. From this old staging post, a short scramble down a set of stairs leads to a lookout – the half-way point in the walk. Perched on the edge of the Nattai Gorge, the lookout boasts an expansive view to the Blue Mountain peaks of Guouogang (1289 metres) and Cloudmaker (1163 metres) on the north-west horizon. With the latter trying hard to live up to its name, we have a quick snack and start on the return trek. Serendipitously, just as we round a bend and see the dark hole in the sandstone wall, the heavens open. We scurry into the tunnel for yet another game of choo choos. Thankfully, it’s only a passing shower and by the time we emerge from the darkness, the sun is shining again.

You’d be hard pressed finding a walking track with such an easy gradient, unusual history, and varied vegetation that leads to such an expansive lookout. Throw in a koala or two and a knock-out tunnel and you have one of the best family bushwalks in our region.

Fact File

Box Vale Walking Track: The start of the track is about 250 metres off the old Hume Highway at Welby (about a two-hour drive north of Canberra). The turn-off for the trackhead is signposted and is 3.7 kilometres west of Mittagong and 0.8 kilometres east of the Wombeyan Caves Road. Although the surrounding terrain is steep and rocky, the track itself is relatively flat and the return walk (8.8 kilometres in total) takes about three hours (or longer if you play choo choos in the tunnel for too long!).

Make a day of it: Picnic tables are located at both the car park and at the lookout. BBQs and toilets are located just to the east of the car park.