The manager of the firm’s Edinburgh base told workers: “When you book a test, please put down that you have at lest one symptom so you actually get a test done.”
An insider said the email from service centre manager Colin Lusted was "telling us to make up we had symptoms to get a test”.
It was sent at least ten days after the company closed its Edinburgh operation following a Covid outbreak.
Rail Gourmet has now conceded the instruction should not have been issued.
Its spokesperson told The Scotsman: “We fully recognise that on this occasion, the wrong advice was given to our team.
"We will be reinforcing the correct protocols to our colleagues.
"Their health, and the health of our customers, remains our priority.”
The Rail Maritime and Transport union (RMT) expressed outrage at the instruction.
Scottish organiser Mick Hogg said: “It is a clear admission by the management expecting employees to tell lies in order to obtain a test
"This is absolutely disgraceful and our members will be seeking legal remedy.”
At least 12 of Rail Gourmet’s 60 Edinburgh staff contracted the virus in the outbreak, which led to the base being closed more than two weeks ago.
Staff were told to get tested and self isloate.
The company’s email was issued on Monday.
The RMT has claimed there were no control measures in place at Rail Gourmet’s Edinburgh operation such as distancing, and a lack of personal protective equipment.
But the company insisted it had followed “strict safety measures” regarding Covid-19.
It said that following the outbreak, “we swiftly contacted our local health authority, took action to temporarily close the unit, and all colleagues who had been working the same shift were advised to self-isolate and arrange to be tested.”
Rail Gourmet, which took over British Rail’s train catering when it was privatised in 1995, also supplies operators such as TransPennine Express, Great Western Railway and Grand Central.
Another firm, DHL, supplies catering to Avanti West Coast, which operates trains between Glasgow, Edinburgh and London.