His most significant achievement (my personal opinion) was construction of Konkan Railway. Not only for being the 1st BOT project of railways, but also the tehnically difficult feat of constructing a railway on difficult geography involving innumerable bridges & tunnels in a landslide-prone region. This reduced the transit time of people travelling from North/West India to the southern parts, as well as made railways accessible to a significant population. One of the more innovative (commercial) features is transporting trucks on rakes!
India has a history of successful engineering feats over the years, so why is Dr. Sreedharan's achievements significant? Firstly, he was made the MD of KRC after his retirement, at a time when most people his age would have been taking it easy after a lifetime of hard work. The commercial acumen displayed was atypical of a retiring government employee in late '80s & early '90s.
Similarly, the delhi metro was also a difficult task considering that it was being constructed in the capital, where out of every 4 people, one is a VIP, another works for a VIP, still another one is related to a VIP while the 4th is neighbour to a VIP. In such an environment, to dig roads, create & manage traffic diversions, attend to the media glare & finish the project within the allocated budget & as per schedule is a near-impossible task that he accomplished. For reference, one may look at the CWG-related projects...
I am reminded of the adage "hard work never killed anyone" when I realise that Dr. Sreedharan is around 80 years!!
Wiki tells me that Dr. Sreedharan was also in charge of the Calcutta Metro... Dr. Sreedharan seems to have been involved with the major infrastructure development in India. His achievements made all indians proud.
The best new year gift that Government of India can give its citizens is award Bharat Ratna to Dr. Sreedharan. He is truly one.