IRCON and L&T are civil construction companies who will build viaducts per Japanese specs as mandated by your NHSRCL. They are civil contractors.
No High Speed bullet train exists in India currently so no Indian company has any experience in building such infra or trackage to spec. When the Japanese implement ToT - then IRCON and L&T will be partners in it for the first time. Period. Building Metros and Bullet trains are two different technologies altogether.
Gatiman Express and Bande Bharat Express are not Bullet trains. They are normal electric or diesel-electric motor propulsion locos with German 1980's coach technology added.
Bullet trains are a different beast than what exist in India currently. Bullet trains are run usually at 350 KMPH or so, Indian speeds could vary somewhat, may be same, maybe a bit lower. All working parts for bullet trains are different to cater to these high speeds. meaning pantographs, wheel design, suspension, bogie articulation etc. This also includes ballastless trackage on viaducts and super elevation of the way track will be laid etc. which will extend to routine (daily) maintenance. Shinkansen trackage is checked/maintained several times each day. This is the reason accidents are minimal. What happens in India WRT maintenance remains to be seen.
I don't get why it is so difficult for Indians to admit that their country is not as developed in rail infra as China or Japan are. These are facts - as plain to see as the nose on one's face. It's okay to admit that Rome will not be built in a day.
This is what happens when bhakts start discussing technology. Way out of their league.