Before the Loop was constructed, Flinders Street and Spencer Street (now called Southern Cross) stations were connected only by the four track Flinders Street Viaduct beside the Yarra River.
The suburban terminus of Flinders Street had become seriously congested by the 1970s, with a throughput of only ten trains per platform per hour (roughly 1,700 trains a day) — compared to a maximum of 24 if there was through running.
Many trains were through routed from the southern and eastern suburbs through to the north and west, but the flow was imbalanced and a number of trains were required to reverse direction.
The Epping and Hurstbridge lines stood alone from the rest of the network, having Princes Bridge station for their own exclusive use.

Several plans had been proposed over the preceding decades to alleviate the bottleneck.
The one that was adopted was the building of a circular railway allowing trains to continue past Flinders Street, turn around and return to the suburbs.
It was expected to boost platform capacity, allowing more trains per platform per hour on the same number of Flinders Street platforms.
The loop would also bring train commuters directly into the northern and eastern sections of the CBD, delivering workers closer to their offices, students closer to RMIT University, and government officials directly to the Parliament buildings.
Although the city's tram network already covered the CBD extensively, trams are not as efficient as trains when bringing large numbers of commuters into the city.
Before the Loop was constructed, Flinders Street and Spencer Street (now called Southern Cross) stations were connected only by the four track Flinders Street Viaduct beside the Yarra River. The suburban terminus of Flinders Street had become seriously congested by the 1970s, with a throughput of only ten trains per platform per hour (roughly 1,700 trains a day) — compared to a maximum of 24 if there was through running. Many trains were through routed from the southern and eastern suburbs through to the north and west, but the flow was imbalanced and a number of trains were required to reverse direction. The Epping and Hurstbridge lines stood alone from the rest of the network, having Princes Bridge station for their own exclusive use.
Several plans had been proposed over the preceding decades to alleviate the bottleneck. The one that was adopted was the building of a circular railway allowing trains to continue past Flinders Street, turn around and return to the suburbs. It was expected to boost platform capacity, allowing more trains per platform per hour on the same number of Flinders Street platforms. The loop would also bring train commuters directly into the northern and eastern sections of the CBD, delivering workers closer to their offices, students closer to RMIT University, and government officials directly to the Parliament buildings. Although the city's tram network already covered the CBD extensively, trams are not as efficient as trains when bringing large numbers of commuters into the city.
City Loop