The British Association for Shooting and Conservation (BASC) spoke out last week against a ScotRail ban on all firearms on its train.
ScotRail said it introduced the ban for safety reasons.
Rupert Soames tweeted at the weekend that lawful guns could be taken on the Serco-run sleeper service.
Mr Soames is Serco's CEO.
The company has confirmed the rules on firearms on the Caledonian Sleeper, which travels between London and destinations in Scotland including Aberdeen and Inverness.
A spokesman said passengers were allowed to travel with licensed firearms if they had contacted the sleeper's guest service centre and a letter of authority had been issued.
He added: "They are allowed to carry them in their cabins and must remain in their cabins with the firearms at all times."
BASC has argued that a ban on the transporting of unloaded, lawful firearms on trains would affect Scotland's fieldsports tourism industry, and harm the Glorious Twelfth.
Chairman Peter Glenser said: "The operators of the Caledonian Sleeper clearly recognise the benefits to their brand and Scotland of welcoming responsible, safety-conscious shooters on their trains.
"This service takes passengers direct into the heart of rural Scotland and into the areas that rely on fieldsports tourism.
"It is estimated that Â£155m per year is injected into Scotland's economy by country sports tourism."
ScotRail said the decision to introduce the ban was made for safety reasons after a licensed firearm was left unattended on one of its trains earlier this year.
The UK's largest shooting organisation has welcomed a public statement from a Caledonian Sleeper boss on the transporting of licensed firearms.