The gunmen only stole about pounds 50 in the late-night attack. Video pictures of the assailants were released yesterday and a pounds 1,000 reward for the their capture has been offered.
The move is seen as a frightening escalation in the level of violence on the railway system. Attacks involving firearms on the Underground, however, remain extremely rare. The number of robberies on the Tube has gone down by 3 per cent to 560 in the last year.
The robbery happened on Saturday night in north-west London but details were only released yesterday because the police have had problems in interviewing the victims - six of whom are from Ethiopia, Somalia and Uganda, one from Singapore and the eighth has yet to be contacted.
The women, all in their 20s and 30s, had been working on late shifts doing casual work.
The robbers were first seen at about 11pm by the Singaporean women on the Bakerloo Line train at Harrow and Wealdstone.
The two men, who were photographed by Underground surveillance cameras, were joined on the platform by two teenage girls who then left the station.
The suspects got on the empty southbound train with the victim. As the train left the station the two men pulled down dark coloured woollen balaclavas and drew handguns. They took about pounds 4 in loose change before the woman was allowed to leave at the next station.
Further along the line at Stonebridge Park station, seven women boarded the train. The men robbed them of about pounds 50 before fleeing at Willesden Junction.
Detective Inspector Alan Pacey, of the British Transport Police, who is heading the investigation into the robberies, said: "We are concerned there could be repeated incidents although there is nothing to suggest that the Underground was targeted specifically.
"It is extremely rare for guns to be used on the Underground."
The rail unions have argued that cuts in staff have made the Underground system less safe. The tube train involved in the latest robbery did not have a guard and the driver was unaware of the attacks.
The use of firearms is rare but gangs of youths have used a technique called "steaming" to move through carriages at great speed snatching bags and wallets. One of the most serious incidents involving a firearm on the London Underground happened in July 1995 when a gunman shot a tube passenger after an argument at Stockwell station in south London.
The gun was fired in the victim's face but at the last minute he lifted his arm to defend himself and the bullet was deflected by the metal bangle he was wearing.
Owners of the newly privatised railways have also recognised that security needs to be improved and have recently begun hiring security guards to protect against pickpockets and robbers.
The gunmen involved in the incident on Saturday were described as black and aged 17-20. They were wearing dark puffa-style jackets and dark jeans and were about 5ft 8ins to 5ft 10ins tall.
Anyone who has information about the incident should contact British Transport Police on 0171-380 1400.