Thomas the Flank Engine could be taking lead role

Chris Froome, left, and Geraint Thomas are vying for the title. Photo / AP

A few years ago, after Geraint Thomas had just buried himself once again for Chris Froome at the Tour de France, an enterprising Daily Mirror sub-editor coined the nickname Thomas the Flank Engine for the Welsh rider.

It was the perfect description. Thomas is a diesel engine. His power, honed from years spent riding as a team pursuiter on the track, unquestioned. He has also been the most loyal of lieutenants.

It is why the 32-year-old has been so highly valued by Team Sky all these years; why he is close to agreeing a new deal to stay beyond the end of this season when other teams must surely have been banging at his door. Sky know his worth.

The question now, as he sits on a 1m 39s advantage over teammate Chris Froome heading into the third and final week of this Tour, is whether Thomas is ready to step up to be lead locomotive? Has he got what it takes to be a winner, both physically and mentally?

Thomas is a serial winner of races, of course. A double Olympic track champion, he has also enjoyed success in the spring classics and in big one-week stage races such as Paris-Nice and the Criterium du Dauphiné.

But until now, he has either suffered bad luck - crashing out of last year's Giro d'Italia and Tour de France - or a bad day, the dreaded jours sans when contesting three-week races.

It is why he has been so insistent this week that, for all his stunning success, winning back-to-back summit finishes at La Rosière and then Alpe d'Huez, and ostensibly tightening his grip on the maillot jaune, he still sees Froome as the team's leader.

There was an almost imperceptible shift in that rhetoric yesterday after the top three on GC - Thomas, Froome and Tom Dumoulin (Sunweb) - marked each other out of stage 14 from Saint-Paul-Trois-Chateaux to Mende, Thomas retaining his 1m 39s advantage over Froome, with Dumoulin a further 11s back.

For the first time, there was no explicit mention of Froome as de facto "team leader". Instead, Thomas spoke a bit more like a leader himself, like someone who might actually win the maillot jaune rather than someone who was just keeping it warm for the four-time winner.

Asked how he felt going into the third week of a grand tour in such an enviable position, Thomas spoke of himself as a "leader" and noted that Froome and Dumoulin might be feeling tired after riding the Giro d'Italia in May.

It felt for the first time as if he was really seeing himself as challenging for the yellow jersey. There was no talk of "pulling" for Froome if that was what the team wanted him to do.

Froome, too, appeared unconcerned. He said he felt he was coming into some form and was not worried by Thomas' emergence.

"It's great to be in that position, first and second, we can play off of each other and I imagine that for our rivals, it's making their lives quite difficult, having two guys to watch like that," he said.

It promises to be a fascinating week. Team Sky are doing their best to play it cool. The longer Thomas stays out in front of his friend, the more confidence he is going to take and the louder the debate over Sky's leadership dynamic will become.

After the week of his life, he is beginning to sound like a lead locomotive as well as look like one.