This urban-regional line (S-Bahn) is an addition to the proposed Budapest urban-regional metro. It is an outer tangent line that would link Érd, on line 30 Budapest – Székesfehérvár – Nagykanizsa and line 40 Budapest – Pusztaszabolcs – Pécs, to Ferihegy airport . There it would use the proposed new station at Budapest Ferihegy airport. The line would cross Csepel Island in the Danube, and allow interchange with:
- the HÉV line to Csepel (which might be connected to the future metro line 5).
- the Kelebia line (line 150) at Soroksár. This is a branch of the proposed north-south Budapest S-Bahn line, and would also be served by a short new branch of metro line 5.
- the second branch of the proposed north-south S-Bahn line, to Gyál and Dabas, using part of line 142.
Two parallel rail lines run through Érd (population 60 000), toward Kelenföld station. Approaching Budafok, they converge, on the bank of the Danube, and then turn north into Budafok-Belváros station. The proposed new line would turn across the Danube, just before the bend. It could be connected to one, or both, of the lines from Érd.
On the opposite bank of the river is an industrial zone, a bridge here seems the best solution. The line must then cross Csepel Island towards Soroksár: all route variants are problematic, since this part of the the island is fully urbanised. In the centre of Csepel, the line would definitely be in tunnel: it should allow interchange with the existing HÉV line. Possibly, the HÉV terminal would be relocated.
The east-west roads in Csepel (two are shown as dashed lines) are not wide. A tunnel under this area seems the only option. On the east side of Csepel Island, the tunnel can continue under the eastern channel. The line can surface, and join the alignment toward Soroksár, without problems. A bridge here would be more problematic.
The proposal implies upgrading of the Kelebia line through Soroksár. The proposed Taksony – Budafok connection (see cross-city tunnel), would take all main-line trains off this section of line: double track should be sufficient.
The tangential S-Bahn line would enter Soroksár station from the north. South of Soroksár station, the new line would turn east, along the alignment of a disused rail line, part of it still in open country.
The alignment of the unused line passes under the M5 motorway, and then crosses the Gyál line (the proposed S-Bahn line). The crossing itself is undeveloped, with housing to the south, and major logistics / industrial development to the north, around the motorway junctions.
The alignment continues east to Pestszentlőrinc – the 18th district of Budapest. This section is still in use: the line turns in a semi-circle, to join the main line – line 100. Most trains on this line would be routed through the proposed Ferihegy airport line. The tangent S-Bahn would connect to that line via a tunnel under the runway, to reach a new airport station alongside the second terminal.
There are other possible routes for a southern outer tangent line, in the Budapest region. One is further north, crossing the Danube north of Budafok-Belváros station, crossing Csepel Island, and then following the line of the Határ út, to enter Kőbánya-Kispest Station from the west. However, that route would not serve the urban centre of Csepel, and would tend to duplicate the proposed southern tangent S-Bahn line.