No need to be too pedantic here. It is understood that by 'capital' they just mean a station that serves the capital.
Howrah isn't the capital of West Bengal, but historically, it has been a rail head for Calcutta.
Dispur...
more... is a suburb of Guwahati, hence the Raj ran initially to Guwahati.
Gandhinagar did not have a railway line until about 2000 and even after that, due to the metre gauge network around Ahmedabad, through trains via Gandhi nagar were not possible. Ahmedabad is very close to the capital and has been a major railway junction for decades, so naturally Raj was started from Ahmedabad.
DBRG and BSP Rajdhanis were supposed to be temporary services, to be run to the actual capitals later, but still run to this day because of operational/political reasons.
By and large IR sticks to the definition of Raj.