The former Conservative transport minister said coaches with windows extending to the roof should run on the “Far North Line”, from Inverness to Wick and Thurso.
The call came as the Scottish Government revealed it had received “very exciting proposals” for planned “scenic trains” among bids for the ScotRail franchise from next year.
Campaigners said glass-roofed trains would also enhance other routes, such as the Glasgow to Oban and Mallaig line, which has twice been voted the world’s most scenic.
Portillo told RAIL magazine: “If companies got together, there are an awful lot of places in Britain where glass-roofed trains could be popular.
“When you’re in Switzerland, and a glass-roofed train comes into the platform, you know you’re in for a treat.”
Such trains include Switzerland’s Glacier Express between Zermatt and St Moritz and the Bernina Express from Tirano in Italy to St Moritz.
The Friends of the Far North Line said glass-roofed trains would also improve passengers’ view of wildlife.
Spokesman Richard Ardern said: “I have thought this would be a good idea for some time, especially since high-backed seats started becoming the norm, thus restricting views down the carriage.
“An added bonus on the Far North Line would be the likelihood of being able to see our iconic red kites and other birds of prey and seabirds.
“Then there are some of the wonderful sunsets you get in the west of Scotland.”
Ardern said stretches that would particularly benefit included under hills along the Dornoch Firth and along the “wonderful” Brora to Helmsdale coastal strip.
He said another spot with dramatic views was Raven’s Rock gorge, to the north of Strathpeffer on the Inverness to Kyle of Lochalsh line.
Kat Jones, of the RSPB, who has travelled on the Glacier Express, said: “It would be absolutely fantastic in Scotland, so long as there wasn’t an extra charge like in Switzerland.
“It would provide an amazing, all-round view out of both sides of the train because you don’t get a sense of the steepness of the hillside in places like the Pass of Brander [on the Oban line], and Killiecrankie [near Pitlochry].”
However, campaign group Railfuture Scotland said improving the view from each end of the train was a better option.
Research officer Ken Sutherland said: “Providing front/rear end views would probably have greater priority benefits in delivering an improved ‘passenger experience’ on trains operating over Scotland’s scenic/tourist lines. The panoramic front/rear end window views offered from the trains operating on Strathclyde’s electrified lines in the 1960s were a major selling point for many passengers, but regrettably ended with later refurbishment.”
Portillo’s call chimes with the Scottish Government’s Transport Scotland agency requiring bidders for the new ScotRail franchise to provide “scenic trains” for such routes. The winner will be announced in the autumn.
These could include “enhanced observation opportunities for passengers” and trees being cleared beside the lines.
A Transport Scotland spokesman said: “Train enthusiasts the world over appreciate the beauty of Scotland’s extensive rural network and we are always keen to hear the views of those who want to experience it at its best.
“Scotland can proudly boast some of the most scenic railway journeys in the world and we are keen to exploit the tourism opportunities this affords, which is why it is a requirement of the next ScotRail franchise to include, at the very least, scenic train provision on the West Highland Line to Oban/Fort William and the Kyle Line.”