It was the latest and by far the most murderous act of terror to have erupted in Pakistan in the past few days, all of them blamed by Pakistan's government on Raw, the Research & Analysis Wing of India's secret service.
India has ridiculed the allegations, but tensions between the world's two newest nuclear weapons states have risen to dangerous new levels. India is now braced for acts of Pakistani terror in return.
A proxy terrorism war may be on the verge of turning into something far worse. The Pakistani foreign minister, Gohur Ayub Khan, and Atal Vajpayee, who is both Prime Minister and foreign minister of India, have been trading words of peace over the past week.
Tension between these two deadly enemies, which appeared to be on the wane during the statesmanlike premiership of Inder Gujral, rose markedly with the election victory in February of the avowedly anti-Pakistani BJP. Now the threats and menaces that have been traded since India's nuclear test at Pokharan on 11 May appear to make further escalation inevitable.
The proxy war between India and Pakistan broke out in Kashmir in 1989 and has been going on ever since. Now it has flared up again - or at least that is how the Pakistani government is explaining the seemingly random bomb attacks that have struck Pakistan in the past few days. And if the string of bombings precipitates tit-for-tat violence in India, the proxy war will be back in earnest.
Within hours of yesterday's train explosion, Pakistan's minister of information, Musashid Hussain, accused India of starting a terrorist campaign on Pakistani soil. "It is the latest in a series of attacks," he told the BBC, "an attempt to destabilise Pakistan following the nuclear tests." He claimed that after an earlier time bomb went off on Saturday - this one in a cinema in Lahore which killed three people - an Indian national had been arrested and has admitted being involved.
Although Mr Hussain's claim that he had "unimpeachable" evidence of Indian involvement in the bombings was treated sceptically, many such bombings in both countries have been attributed to the proxy war during the past nine years.