There is something quietly thrilling about walking a path that was never designed for feet. The line once carried expresses and coal, milk churns and holidaymakers, and now it carries you, a flask and a slightly overexcited dog. Britain is crisscrossed with thousands of miles of disused railway, and much of it makes superb walking: gentle gradients, wide cuttings, deep tunnels and stations that have been reborn as cafés, museums and village halls.

Why old railway lines walk so well

Engineers were ruthless about gradients. Steam locomotives hated hills, so lines were built to creep up valleys at tiny inclines, which means a former trackbed is usually far flatter than any footpath that grew organically across the same landscape. If you are recovering from injury, walking with young children or simply want a long, chatty day out without a lung-busting climb, a disused railway is often the best answer in the county.

They are also generous in width. Where a footpath might be a muddy ribbon between hedges, a trackbed gives you two or three metres of firm going, often surfaced, usually drained. Expect a steady trundle through farmland, with high embankments blocking the view on one side and a sudden, wide reveal on the other.

Reading the landscape as you go

Half the pleasure is detective work. Once you know what to look for, the railway reveals itself everywhere.

• Mileposts and marker stones — small, often moss-covered, sometimes still painted, sitting quietly at the field edge.
• Sleeper impressions — faint parallel lines in the ballast or underfoot, especially on unsurfaced sections.
• Brickwork and coping stones — the ornate edging on bridges and platform edges, often in a local brick that tells you exactly which company built the line.
• Signal posts and telegraph insulators — rusted stumps, occasionally with a ceramic insulator still clinging on.
• Cuttings and spoil heaps — the reversed geology of a cutting, where the spoil was tipped alongside, now often a haven for orchids and butterflies.

Old station names can be misleading. A halt might be a mile from the village it served, and a "junction" may now be a single farmhouse. Take a large-scale map and compare it with a pre-1960s railway atlas; the discrepancies are often the most interesting thing you will find all day.

Tunnels, viaducts and the bits that need care

Tunnels are the headline act, and they demand respect. Many are brick-lined, dripping, pitch dark and home to hibernating bats, which are protected. Some are gated, some are open, and some have a public footpath diverted over the top. Never walk a tunnel without a torch, never enter one that is fenced or signed as closed, and keep your voice down between October and March when bats may be roosting.

Viaducts are usually the finest viewpoints on the route. Where a footpath crosses, stop in the middle and look down: you will often see the river the railway followed, a mill leat, or the curving earthworks of the line as it heads off towards the next town. Bridges over the trackbed are just as good, giving you a clean, elevated view of fields, hedgerows and the odd curious cow.

Niche pastimes for the trackbed

Once the walking itself becomes easy, people start inventing games. These are the ones I see most often, and they are all free.

• Bridge bagging — noting every overbridge, underbridge and occupation crossing on a stretch, with its number if it still has one.
• Station collecting — visiting every surviving station building on a former line, whether it is now a house, a workshop or a tea room.
• Geocaching and letterboxing — many old railways are dense with hidden caches because the hiding places are so good.
• Wildlife recording — railway cuttings are linear nature reserves; slow worms, glow-worms and a startling range of orchids all turn up on them.
• Sketching and photography — the straight lines of a trackbed give you a vanishing point for free.

Practical kit and a few cautions

Wear proper shoes. Surfaced trails are fine in trainers, but unsurfaced trackbed can be loose ballast, which rolls underfoot. Take a torch even on a short walk, in case the path ducks under a low bridge. A paper map is worth carrying, because signal is patchy in deep cuttings.

Check access before you go. Many former lines are public rights of way or permissive paths, but some sections are private, some are leased to heritage railways, and some are closed for engineering work or nesting season. Look for local authority trail leaflets, parish noticeboards and walking group websites — the detail on them is usually more accurate than anything else.

Coffee at the old station

The best endings involve a platform. Converted stations along disused lines have become some of the country's most charming cafés, often with the original canopy, a nameboard and a bench where the waiting room used to be. Order a pot of tea, sit where the express once thundered through, and watch walkers, cyclists and the occasional horse rider drift past on the line you have just followed. It is a very British sort of afternoon: a lost railway, a flat mile or two, and a slice of cake at the buffers.