Bob Crow, leader of the RMT rail union, claimed the stand-in executives had worked up to 16 consecutive hours to learn basic duties. Operators, he said, were displaying a "stunning disregard for public safety" to undermine walkouts by up to 3,000 guards on nine of the network's 25 companies in a dispute over safety.
"They are bending the rules beyond recognition in their effort to run some trains - and that underlines our fears over their attitude to safety," said Mr Crow.
"Hastily and inadequately trained stand-ins cannot possibly do the job of properly trained guards."
The RMT intends to call further strikes if today's stoppage and action scheduled for Monday and April 17 fail to resolve a long-running dispute.
As few as one in 10 services will survive on Virgin Cross Country and Central Trains routes as guards, complaining that changes to rule books will turn them into "Kit Kat-sellers", severely disrupt timetables.
The Association of Train Operating Companies said managers had received the same training as guards, typically seven days of instruction.
The only difference, a spokesman said, was over routes with managers being taught only the lines they would work on during strikes.
"Any manager is undergoing identical training to that of the guards, the only variation being that of routes because they will work only on specified routes," he said.
Disruption to rail services
Arriva Merseyside About half of the services will be cancelled, although the company said it would increase the number of seats. The frequency of services will alter - from every 15 minutes to half hourly. Where possible the number of carriages will be doubled.
Connex About 70% of its services will run, but there will be more disruption to longer distance services, with just 30% running. The chief executive, Olivier Brousse, said enough staff would be trained to run all the services in any future strikes.
Central Trains Just 10% of the company's 1,300 daily services will run.
South Central About 70% of trains will run, rising to 98% in greater London and driver-only trains from London to Brighton. Charles Horton, the firm's managing director, said the strike might cause "significant inconvenience" and warned that the trains would be more crowded.
ScotRail Half the 2,000 daily services will not run, with overnight sleeper services between Edinburgh, Glasgow, Aberdeen, Inverness, Fort William and London, being suspended during the industrial action. The firm's managing director, Peter Cotton, said the "unnecessary" strike would force thousands of people on to the roads.
Silverlink Three in four services and half the "county" services between London and Northampton will run, but some routes, including Northampton-Birmingham and Bletchley-Bedford, will have no trains.
Thames Trains Nine in10 trains will run. Cancellations are expected and buses will replace trains on some routes, including Stratford-upon-Avon to Leamington Spa.
Virgin West Coast Half the trains will not run and there will be no direct London-north Wales services.
Virgin Cross Country Nine in 10 of the trains will not run.
