So as per Indian media:
1. 360 Kgs of Ammonium Nitrate and IED material confiscated in Faridabad, Haryana (Dehli NCR) from a rented vehicle one day earlier.
2. The car in which the explosion occurred the following day had a Haryana registration plate and multiple owners were identified with the latest being resident of Pulwama (interestingly the latest owner and the person in whose name car remained registered have Muslim names)
3. The explosion didn't use sharpnels or left any soot marks (meaning no net negative oxygen balance)
Hmmm
1. Well pure Ammonium Nitrate will explode with net positive oxygen balance so won't leave any soot marks.
2. Previously in Pahalgam, they were very quick to ascribe blame, here they were very quick to absolve it. Their Police Commissioner immediately dismissed any connection to the explosive bust in Haryana from a day earlier.
So, in theory they could have loaded the i20 with some amount of busted Ammonium Nitrate (my guess would be around 35-50 kgs of pure NH4NO3 ) - then plotted to have an oblivious driver, drive the car to a location where the route has the Dehli Fort Metro Station signal/crossing on the way, and remotely detonated it. The gas cylinder their police initially reported could be the IED

They could present this as a separate case of terrorism (as most of the busted explosive material is still with them and no one is going to ask them to weigh it) but still somehow connected to and possibly sponsored by Pakistan
Let's see where they fix the blame finally.