Probably the first thing to address with your question is that railway artillery has always been a niche thing. Very specialized for when very big guns were needed to throw shells at targets that were within range of an available rail line. This typically would be to assist in a city assault or siege of a major fortress. The bulk of any army's artillery has always been field pieces that can be maneuvered off rail.

The other thing is that for the last 100 years we've been in an era where mobility is arguably more important to artillery design than weight of shell. Guns from WW I were capable of throwing heavy shell rapidly and relatively accurately, where artillery design excelled during and after WW I was to make field pieces lighter and more mobile. This was in part what allowed a return to war of maneuver in Europe.

It's worth noting that developments in mobility for artillery were a major trend in pre and Napoleonic warfare as well. The major development wasn't making guns bigger or heavier caliber, it was making guns of similar caliber more maneuverable in the field. This had a huge impact on 19th century warfare as artillery went from practically fixed emplacements on the battlefield to often highly mobile guns that could respond rapidly to changing circumstances in the field. For the entire history of artillery development, mobility has been as much a focus as weight of shell or accuracy.

So back to rail artillery, a major problem with it is that it is fixed unless moving along RR tracks. This became increasingly treacherous with the rise of air power. Railway artillery was used in a limited capacity into WW 2, but this became impractical and even untenable with the rise of ground attack aircraft and the importance of air superiority. Railway artillery relies on good track to move, which can be easily blown up by air strikes, but most of all railway artillery is big and it's travel extremely predictable. Both factors made it highly vulnerable to air strikes and even just partisan railway sabotage. It's predictability and limited range of movement became a huge disadvantage.

Looking post WW 2, the emphasis again for artillery was on mobility more than weight of shell. Mobile, sometimes mechanized, gun systems that could keep up with a mechanized advance to provide continuous fire support for armies that quite often were not fighting right next to rail lines.

Today we can look at the war in Ukraine to see how drones, guided munitions, and missiles have made the concept of mobility for artillery very literally a matter of life or death in the modern era. Immobile or predictable artillery cannot survive on the moden battlefield. Shoot and scoot, often in a matter of minutes, is necessary to avoid quick and incredibly accurate counter battery fire. Once again, a heavy railway gun easily discovered and targeted would not stand a chance of surviving long enough to justify its deployment.

Finally, a lot of the purpose of rail artillery had been replaced by air power and guided missiles. The main role of rail arty was to provide a powerful long range striking ability. Long range precision guided missiles and bombs can do that far better than a limited asset such as railway artillery. The modern missile is longer ranges, more mobile, and more accurate compared to a giant field piece on rail.

A couple good sources in this: Firepower: How Weapons Shaped War, by Lockhart, and The Arms of Krupp, by Manchester.