The Carlingford Line had been partly electrified up to Rosehill in 1936, with the rest of the line through to Carlingford Station being converted to electric in 1959. The line between Rosehill and Carlingford consisted of a single track with no passing loops and also includes a station for the UWS Parramatta campus. As a result of having only one track, services on the line remain extremely limited, with only a single 3 carriage service per hour operating as a shuttle between Clyde and Carlingford.
A number of proposals have been made over the years involving this line:
1. The line would form the bulk of a future Parramatta to Epping Rail Link, with underground portions linking Epping to Carlingford and Parramatta to Camellia. The line would be upgraded to dual track as part of this proposal. This idea has been seriously floated as far back as the Carr Labor government’s 1998 Action for Transport plan and most recently by the Keneally Labor government’s 2010 Metropolitan Transport Plan that would complete the Parramatta to Chatswood connection originally proposed in the 1998 plan. The election of the O’Farrell Liberal government in 2011 put this plan on ice, choosing to focus instead on the Northwest Rail Link.
2. The construction of a passing loop was initially included as part of the Cityrail Clearways program to increase capacity on existing lines, before being quietly dropped. Such a passing loop would probably double the capacity of the line to 2 services per hour, but without the expensive exercise of building double track the whole way along the line.
3. UTS academic Garry Glazebrook has suggested converting the line to light rail and for the Southern end to link up to Parramatta rather than Clyde. Given the low capacity nature of this line, it would allow for frequent services along this line that would take commits directly to Parramatta, forming the base of a future Western Sydney light rail network. Doing so would require any Parramatta to Epping Rail Link to go entirely underground, probably following an alignment along Pennant Hills Road.