Rio's subway extension is set to open four days before the Opening Ceremony ©Getty Images

Parts of the Rio de Janeiro subway network are set to be open until either 1 or 2 o'clock in the morning during the Olympic and Paralympic Games in a move designed to ease concerns over the city transport system.

This comes after State Secretary of Transport Rodrigo Vieira told Globo how the final installation of the line four extension westwards to link the city centre with the main Games-hub at Barra de Tijuca was completed over the weekend.

Overall progress is now said to be 90 per cent complete.

A wagon was reportedly pulled by another vehicle to test the new line, passing five stations on the route of which the construction is still being completed.

After repeated construction delays, it is now expected to become fully operational on August 1 - just four days before the Olympic Opening Ceremony.

It will only be a "soft" opening, however, with the line set to be restricted to spectators with tickets and accredited personnel rather than the general public.

Opening hours of the new link and of all other lines will be from 6am to 1am Monday to Saturday during Games-time, and from 7am to 1am on Sundays and holidays.

It may be necessary for some lines to remain open even later due to the late finishing times for some events - including swimming finals at the Olympic Aquatics Stadium in Barra.

Trains are expected to operate on line four every eight minutes.

The line extension is a major legacy project ©Rio 2016
The line extension is a major legacy project ©Rio 2016

The extension to Metro Linha 4 westwards beyond Ipanema Bay is among the most important legacy projects of the Games, intended to improve the commute for more than 300,000 people a day.

Although not part of the initial Rio 2016 bid, it is now seen as a vital part of the Games-time transport plan, acting in conjunction with BRT to transport thousands of spectators to the Olympic Park in Barra de Tijuca.

Most venues in the Copacabana and Maracanã clusters are accessible by subway, while train extensions will be required to reach the second major Games hub at Deodoro.

Current plans are for the extended line to be rolled-out to the entire population on September 19, where it will operate at first from just 11am until 3pm.

The project has been plagued by delays, with concerns peaking in February when a leaked email sent by Rio de Janeiro Mayor Eduardo Paes to the International Olympic Committee (IOC) warned of a "high-risk" it will not be ready in time.

He suggested they should consider alternative solutions.

Problems were exacerbated by financial woes affecting the State Government responsible for development work, as well as the harsh rocky terrain the line needs to navigate.

A "major breakthrough" was claimed in April, with constructors breaking through the final rock wall to complete a connected tunnel system throughout the length of the line.

"If the line is not open, or if there is a break-down during the Games, more buses would be organised to pick up spectators," said the International Olympic Committee's (IOC) leading Rio 2016 advisor Gilbert Felli in October.