The Sub-Balkan Line — Sofia to Burgas through Bulgaria's Longest Tunnel

There are two ways to reach the Black Sea from Sofia by train. One runs east through Plovdiv along the Thracian plain, which is the way most people go and the way the map suggests. The other turns north-east out of the capital, climbs into the valley between the Balkan Mountains and the Sredna Gora, and disappears into a hole in the hillside above Koprivshtitsa.

That hole is the Koznitsa tunnel: 5,808.5 metres under the Koznitsa ridge, opened on 6 June 1951, and still the longest railway tunnel in Bulgaria. For most of its working life it was also the thing that made this route slow. The bore had not been rebuilt in nearly fifty years, and trains crossed it at 25 km/h — a walking pace held for more than five kilometres, every single service, in both directions.

It was rebuilt in 2021. The line's speed limit through the tunnel went up to 100 km/h, and the consequence is visible in the timetable rather than in the ground. The Sub-Balkan route is 418 kilometres from Sofia to Burgas against 450 by way of Plovdiv, and today it carries the fastest train between the two cities — five hours and forty-nine minutes, against six hours forty-two by the plain. The mountain way is now the quick way.

What the ride is actually like

None of that is why you would take it. You would take it because it is one of the loveliest stretches of everyday railway in the Balkans, and it costs somewhere around twelve to fourteen euros in second class.

The train spends its first hour crossing the flat country east of Sofia, then begins to climb through Zlatitsa and Pirdop with the Balkan range rising steadily on the left. At Koprivshtitsa it stops in a river gorge with nothing around it but forest — the town it serves is nine kilometres away over the hill. Then the tunnel, and on the far side the whole character of the journey changes: the line comes out into the Stryama valley, drops down past Klisura on stone viaducts, and runs the length of the Rose Valley with the mountains on one side the entire way to Kazanlak. After Sliven it lets go of the hills, crosses the plain to Karnobat and reaches the sea at Burgas in the evening.

It is an ordinary BDŽ service — no heritage stock, no dome cars, no premium. Three trains a day each way, a compulsory seat reservation costing 51 cents, and some of the best mountain scenery in the country going past the window for the price of a sandwich.

The Sub-Balkan line is Bulgaria's Main Railway Line 3, and it took forty-five years to finish. Its outer ends came first, built as ordinary lowland railways: Zimnitsa to Karnobat opened on 23 May 1907, Tulovo to Kazanlak on 21 November 1921, Dabovo to Tvarditsa in 1928, Tvarditsa to Sliven in 1930, and the western end from Iliyantsi to Makotsevo on 6 December 1931.

What stood between them was the mountains. The section from Makotsevo to Sopot was surveyed and re-surveyed for close to thirty years before a final alignment was adopted in 1940, and it was built piecemeal after that: Karlovo to Kazanlak on 3 October 1939, Karlovo to Sopot that November, Sopot to Klisura on 14 September 1947, Makotsevo to Dolno Kamartsi in 1949. The last piece went in last. A first train ran the whole way on 7 September 1952, and the complete Sofia–Karlovo–Karnobat–Burgas line was opened on 21 December 1952 — the country's third east–west artery.

The tunnel that held it up

The engineering that mattered is under the Koznitsa ridge, where the Balkan Mountains meet the Sredna Gora. The Koznitsa tunnel runs 5,808.5 metres between the stations of Koprivshtitsa and Stryama, single track, rising to a highest point of 818 metres. It was finished on 6 June 1951 and has been the longest railway tunnel in the country ever since; the Ministry of Transport calls it the longest on the Balkan peninsula. The line's second-longest bore, Galabets at 3,030 metres, was completed five months later.

Electrification came much later and in stages — Sofia to Karlovo in 1968, Karlovo to Tulovo in 1969, on to Sliven by 1971 — and the tunnel was wired with the rest of it. Then it was left alone. By the 2010s it had gone, in the ministry's own words, close to fifty years without reconstruction, and the speed through it had been cut to 25 km/h.

The rebuild was inspected and signed off on 23 August 2021. It cost 29,029,775.15 leva including VAT, renewed 5,812 metres of track bed, built drainage, laid partial waterproofing, replaced the overhead line and put lighting and video surveillance into the bore. The permitted speed went back up to 100 km/h.

Sofia to the tunnel

Trains leave Sofia Central and thread the eastern suburbs — Poduyane, Iskarsko Shose, Kazichene — before the country opens out. This is the Zlatitsa–Pirdop valley, a wide agricultural trough with the Balkan Mountains filling the windows on the left and the softer Sredna Gora on the right. Zlatitsa and Pirdop are the only substantial stops.

Thirteen minutes past Pirdop the line turns into the gorge of the Topolnitsa and stops at Koprivshtitsa, a 1951 station building by the architect Vello Debelyanov standing alone among wooded hills. Immediately beyond it the train enters the Koznitsa tunnel.

The Rose Valley and the run to the sea

Out the far side is Stryama, then Klisura, where the railway crosses the valley on a tall stone viaduct above the roofs of the town. From here to Kazanlak the train runs the length of the Rose Valley, the strip of country between the two ranges where most of the world's rose oil is grown, calling at Karlovo — Vasil Levski's birthplace, at 452 metres — then Kalofer, Pavel Banya and Kazanlak.

The mountains stay alongside as far as Sliven, where the Blue Rocks rise straight out of the town. After that the line lets go: Karnobat, Aytos, the flat Burgas plain, and the terminus beside the port at Burgas.

Three fast trains a day run each way over the Sub-Balkan line, and all six are ICF — BDŽ's Intercity Fast category, on which a seat reservation is compulsory. The times below are from BDŽ's own live timetable at razpisanie.bdz.bg, read for Friday 14 August 2026.

Sofia to Burgas

Burgas to Sofia

Only four of the six stop at Koprivshtitsa. The 06:10 from Sofia and the 16:25 from Burgas run straight through it, and on the day checked BDŽ's planner answered a Sofia–Koprivshtitsa query with the 06:10 by routing it to Karlovo and back again — a three-and-a-half-hour version of a journey the 13:15 does in 1 h 44.

The slower way, and the other route

Regional trains also serve Koprivshtitsa from Sofia, taking about two to two and a half hours instead of an hour and three-quarters, with no reservation needed: 05:25, 07:55, 13:30, 18:00 and 19:15 on the day checked.

The Sub-Balkan line is not the only way from Sofia to Burgas. Trains numbered in the 8000s go via Plovdiv and the Thracian plain — 450 km against 418 — and the quickest of those, ICF 8611 at 06:30, takes 6 h 42. BDŽ's own board marks the 06:10 over the mountains as the fastest trip of the day.

Before travelling

BDŽ prints a standing notice on its timetable pages: because of repair works in various sections of the railway network, there may be changes in the trains' schedules, and it asks passengers holding a ticket bought in advance to check the schedule again on the day of travel. The live timetable is the authority, and it is worth a look the morning you go.

One quirk of that timetable is worth knowing: it answers for the next day or two and nothing beyond. A query for a date weeks ahead returns no trains at all, which reflects the planner's horizon rather than any gap in the service.

Times as of August 2026, from BDŽ's live timetable — razpisanie.bdz.bg (opens in a new tab).

This is an ordinary domestic BDŽ service, priced like one, in euros since Bulgaria adopted them on 1 January 2026. The fare is not a fixed figure: BDŽ carries a calendar fare alongside its standard one and warns that prices vary by day. Read a day apart in August 2026, the same second-class Sofia to Burgas fare came back as €14.32 and then €11.71 — so the honest answer is somewhere in the region of twelve to fourteen euros for 418 kilometres. Sofia to Koprivshtitsa held steady at €5.11, which is the whole climb into the mountains for about the price of a sandwich.

Every ICF train on this route carries a compulsory reservation at €0.51, so a specific seat is allotted rather than left to chance. There is no sleeping car on the Sub-Balkan trains; the overnight services go the other way, via Plovdiv.

Bulgaria is covered by Interrail and Eurail passes, and the national operator BDŽ is the only railway they need to cover here — every train on the Sub-Balkan line is a BDŽ service.

Whether a pass is worth it for this journey

Almost certainly not. A pass earns its keep where point-to-point fares are high, and Bulgarian fares are among the lowest in Europe: the full 418 km from Sofia to Burgas over this line came back at between €11.71 and €14.32 in second class on two readings a day apart, and Sofia to Koprivshtitsa at €5.11. The Interrail Bulgaria Pass, second class, is €67 for three travel days within a month for an adult and €50 for a youth. It would take four or five long journeys inside its validity before the arithmetic turned in its favour.

Where a pass does make sense is as part of a wider European trip that happens to pass through Bulgaria — a Global Pass being used from Vienna or Belgrade, say, with this run to the Black Sea as one leg among many. In that case the pass is already paid for and the line costs nothing extra.

Reservations still apply

A pass does not remove the compulsory reservation. Interrail's own country page for Bulgaria says that reservations are required on the faster express services and puts the charge at approximately €0.25 (50 stotinki in the old currency); BDŽ's timetable shows €0.51 for the ICF trains on this route. Either way it is a trivial sum, but it has to be bought — at a Bulgarian station counter — before boarding.

The slower regional trains that also reach Koprivshtitsa carry no reservation requirement at all, which is one practical reason a pass-holder might prefer them.

Pass conditions as of August 2026, from interrail.com (opens in a new tab) and BDŽ's timetable.

The line's stops are not incidental to it. Koprivshtitsa is where the April Uprising began, Karlovo is where Vasil Levski was born, Kazanlak sits on top of a Thracian royal necropolis, and Sliven has a mountain rearing out of its northern edge. All of them are reached on the same twelve-to-fourteen-euro ticket, and any of them will hold a day.

One caution about the first: Koprivshtitsa station is nine kilometres from Koprivshtitsa, on the far side of a ridge, in a river gorge with no town attached. The connection is covered by a shuttle rather than by walking.

The whole line can be done in a day — six hours and change, three trains each way — but the interesting version breaks it. Koprivshtitsa is worth a night on its own, and Kazanlak makes a natural second stop halfway along the Rose Valley, with the Thracian tombs and the rose fields within a short drive.

The four places below sit at the departure, at the two stops most worth breaking for, and at the far end. None of them is booked through this site: each card links to the property's own website, and no price, rating or star count appears on any of them, because none has been verified here.