No one really does it any more and the practice only lasted a around a decade even in the US. Once ICBMs became reliable enough they largely replaced bombers because it was much easier to dig a bunker than to constantly have bombers on standby or in the air and they provided a better strike capability because there's no real defense vs bombers that could be intercepted.
There is defense and it's pre-emptive strike, being static is one of the weakness of such system, that's why there are subs and Russia (S.U.) has some of the nukes on the trucks.
There also was a railway version, SS-24 Scalpel (РТ-23 УТТХ) which was mounted inside always-on-the-move train locomotion. Decommissioned in 2005 but they have new version in development.
There's a distinction between defense and deterrence the former stops something that's been done, eg shooting down missiles and bombers, and the latter in the nuclear context is always maintaining the ability to strike back.
A pre-emptive strike isn't defense it's just starting the war and it also doesn't prevent you from being struck.
Unless you're in Cuba firing at the US or Europe aiming at Russia there's always time to launch a retaliatory strike before the first missiles hit and missile bases were designed to survive really close hits. That's part of why the US freaked out so much about missiles coming to Cuba and why Russia developed more mobile launch capability than the US. The only real 'defense' against and ICBM (at the time we're maybe figuring out real defenses now) was to be able to survive to strike back which isn't so much a defense as deterrent.
