This is a single bore 930ft tunnel carrying a single line. The line was doubled by opening a second single bore tunnel to the immediate south, Cartsburn Tunnel South. The second tunnel is slightly longer at the east end and is now disused.
To the west is Whinhill station, the platform built on the site of the lifted westbound line. To the east the line is crossed by a disused double track bridge of the Greenock and Ayrshire Railway.
To the east of the tunnel (and the Ayrshire line bridge) was Easter Greenock Castle, the ruins of which were partly buried by the railway embankment. (The site was to the south of the embankment, east end of Castle Road.)
Greenock Cathcart Street [1st]
Greenock Princes Pier [1st]
Greenock Princes Pier [2nd]
| Cartsburn Tunnel South|
Balwhirley Tunnel South
Balwhirley Tunnel North
Greenock Works and Shed
Greenock John Street Signal Box
Garvel Dock Branch Junction
|Location names in dark blue are on the same original line.|