Great Western Rail launched the new service on Friday (September 9), which will see an increase the frequency of trains and the number of carriages per train.
John McDonnell said he and many of his constituents had been concerned about overcrowding on trains.
The Labour MP said: “I travel on this route almost daily and like my constituents I have been very concerned about the frequency and overcrowding on trains.
“This is why I was very pleased to be given the opportunity to take part in the launch of this new train service which I've been campaigning for.”
According to the independent Office of Rail and Road over 1.5million passengers travelled from the Hayes and Harlington Station in 2014/15.
The new service will operate on the same path as the old two-car train, but will see an extra 528 seats in the morning rush hour.
There will also be space for another 606 passengers in the evening peaks to and from of London Paddington.
Mr McDonnell, the serving Shadow Chancellor, added: “The new service is more environmentally friendly and also more efficient as it carries more passengers.
“Great Western Rail are hoping to introduce more of these trains over the next few years which is good news for my constituents as these changes have been needed for a very long time.”