The ordeal usually begins somewhere west of Hammersmith. A denim-clad figure, clutching a battered acoustic guitar, steps into the carriage. Tied to a satin scarf around his neck is a microphone. This, you later discover, is connected to a cunningly camouflaged mini-amplifier.
Taking his position in the doorway he waits for the train to pull away before greeting his audience, most of whom are bemused tourists en route to the airport. Behold, the Busker From Hell.
As he launches into his first number - a cruel rendition of something by the Everly Brothers - you begin to wonder whether London buskers were always this brazen. By the end of his third song - probably Streets of London - you firmly believe that summary execution would not be too harsh a punishment for this offence. The most excruciating aspect comes from the knowledge that escape is not a realistic option. True, you could make a dash for one of the adjoining carriages while the train is stationary. But it's an even chance he'll follow you at the next stop, forcing you to sit through his performance all over again.
Then there's the embarrassment factor. If my experience is typical, this will remain high throughout the performance, rise slightly during the inevitably awkward appeal for funds, and reach its toe-curling zenith on the realisation that foreigners may remember this occasion long after you have tried to wipe it from your memory. The discomfort posed by beggars asking for money pales in comparison.
What the Busker From Hell cynically chooses to ignore is that live music in the Underground is generally tolerated because nobody is forced to listen for longer than it takes to walk out of earshot. Sensible buskers understand this to the extent that some are happy to play the same catchy number for hours on end. Many seem genuinely surprised if you even acknowledge their existence.
In the confines of a train carriage, however, the passenger's status is reduced to that of a hostage. He is left no option but to listen, crimson-cheeked, verse after verse; chorus after chorus. It would be far less painful - and more lucrative - if the busker simply issued an ultimatum: 'Cough up now and I'll spare you the Elvis Presley medley.'
Given that it is now much harder for buskers to find a decent pitch and remain there without being moved on, the emergence of the mobile busker was perhaps inevitable. I can even see the logic in specifically targeting homebound tourists who need little excuse to dispose of their loose change.
Ultimately, though, this only serves to undermine the reputation of all buskers, regardless of their talent and sensitivity.
Like most commuters, I have no wish to see busking outlawed where it can be entertaining or easily ignored. But London Underground's drive to stop buskers performing on trains has to be applauded.
With any luck, the musicians working the Piccadilly Line will one day discover how unwelcome they are. That might persuade them to stay by the escalators or along the walkways.
Until then, there will always be a risk of encountering the Busker From Hell. Unless, of course, you take the safe option and catch a cab to the airport.