The victim, who is from the Clacton area, has not yet been named by police.
However, tributes began flooding onto the social networking site Twitter using the hashtag #RIPJay.
One Twitter user, under the name Mary-Ellen1, said:
RIP Jay my little brother, taken too young. We will get you justice.
RIPJay, the mark you left on this Earth is irreplaceable, the mark you left in our hearts is unforgettable.
More top news
The high of 35C was recorded on a Bakerloo line train between Piccadilly Circus and Oxford Circus at 2pm.
Weather forecasters say temperatures could reach 35C, beating the previous record of 34.6C in the scorching summer of 1976.
After years of political manoeuvring over the future expansion of Heathrow Airport, there has been an important development this lunchtime.