Lord McConnell spoke out about the standard of facilities on a two-carriage train travelling from Stirling into the capital. The 53-year-old used Twitter to voice his displeasure at the lack of air conditioning on Thursday's 9.36am train, tweeting: "Come on #scotrail trains to Edinburgh in August with no windows/aircon and 2 carriages. Get a grip!"
Lord McConnell - a supporter of the Edinburgh Fringe programme - later said he might need an ambulance as "elderly in here almost passing out".
His criticism prompted a rapid response from ScotRail. The company tweeted an apology before leaping to its defence, explaining it was running its biggest-ever festival timetable.
The operator said it was using every train at its disposal to provide at least 330,000 seats on an extra 5469 carriages over the course of the four-week extravaganza. The increased commitment includes later trains and new services. However, rail consultant and author David Spaven said there were issues with the quality of some of ScotRail's trains.
Fares for peak-time travellers are to rise by 3.1% in January. Watchdog body Passenger Focus has said the hike must couple with improvements in services.
ScotRail, whose franchise is up for renewal in 2015, said some services would still be busy as they have a finite supply of trains.