Valid points indeed, but few observations of my own:Impractical.
1.) Placing strategic cannons on rail tracks causes the enemy to incentivize destruction of those rail tracks. Those same rail tracks are also critical in supplying your troops. They are the arteries carrying lifeblood of your military punch. The enemy will zero-in their long range weapons on your rail network. Once these get taken out..................Moreover, you would be increasing the traffic on the rail line, delaying supplies.
2.) The alternative is to build special railway lines that would (in wartime) be used exclusively by these cannons. Laying new tracks and building massive cannons would drive the cost of the entire project above that of a massive Brahmos barrage, low cost of the individual arty shell not withstanding.
3.) If you fire unguided rounds through it, the dispersion would be far too wide to be of any military use. Indian Army is not in the business of causing massive collateral damage to the civilians of the enemy state. This is not World War 2.
4.) If you want to use guided rounds, the electronics need to be hardened to withstand extreme G-forces. India is weak in that research.
Better to invest all that money into Railgun technology and Missile inventories.
1> Branch lines can be cut from existing lines to keep military traffic away from main lines. This includes a whole net of branch lines.
2> Can we create a rapid railroad repair capability using both railways and armed forces personnel?
3> Enemy will need very long range PGMs to be able to target a block of area where the railroad is situated to be able to destroy part of it. I don't expect a Paki aircraft to come in 200 km inside Rajasthan to hit one of these tracks or a Chini aircraft to come deep into Punjab or Uttar Pradesh.
4> Rounds fired from such cannons should be PGMs only. Given how far we have progressed in PGM tech, we shouldn't doubt our capabilities to produce something like an excalibur or better. FYI that research is already ongoing. Just because we are weak we should not give up.
5> Missile inventories are being built up well thankfully. Railgun research is certainly on, but railgun need a major power source to tag along with. Thats why for now ships will make a good choice to host them.