When I used to work on the railways in the UK, I worked on electric locomotives that were built in the 1960s and whose control circuits were largely mechanical relays. Many were "open frame", which means that they have no case on them and the number one cause of failure, while I was there in the early 1990s was relay failure.
Like any complex system, it often only takes one small part to fail to make the entire system fail and in this instance, a failure on the main line would require a replacement loco, often a tow to the nearest maintenance depot, which could be 50 miles away or more and the parts to diagnose and repair it. Basically a royal pain the backside.
One thing I often wondered was why they didn't replace the mechanical relays with solid-state relays. These were relatively new so I didn't expect the original locos to use them or indeed to retrofit every loco but why not perform the experiment on one locomotive and see whether they work - they should do right?
What is a solid-state relay (SSR)?
Solid-state simply means that the state changes without any mechanical parts and a solid state relay, although designed to look like a simple "black box" is actually an electronic circuit which involves a power switching device (usually a MOSFET, SCR or TRIAC) and an opto-coupler so that energising the "coil" input would trigger the power device and switch on the main load.
It sounds good so far, even with a few additional components to avoid floating inputs and perhaps zero-crossing detection to avoid electronic noise, the circuit is relatively straight-forward and no moving parts means reliability right?
Yes, reliability is higher so why not use them in every scenario that a mechanical relay is used in?
Firstly, SSRs are better in several ways compared to mechanical relays. As well as that already mentioned, their switching times are much faster (not necessarily a big deal for relay circuits), they are much lower electrical noise because there are no arcing contacts to generate EMI, they have no contacts to oxidise and therefore their resistance to increase over time leading to burnout, they no have mechanical adjustments to screw-up (I once caused a failure because I replaced some contactor tips and didn't leave them straight! They got stuck.). They are generally smaller (although they might require heatsinks) and much more immune to unfriendly environments such as vibration and noise-sensitive environments. They are basically sweet!
So why wouldn't we use them?
Two main reasons.
Firstly, they are still generally more expensive than mechanical relays, which is a shame because there is no magic inside it, just a switching circuit. Considering the longer lifespan, however, the price difference is not likely to be a major issue except in high current circuits.
The second more significant problem comes from the fact that mechanical relays are volt-free contacts and SSRs are power electronics devices. We are really asking "why a MOSFET instead of a relay", which makes the differences more obvious. SSRs can be more fussy about polarity and whether they support AC or not; they do not have effectively infinite open circuit resistance; they do not have closed resistance as low as a (new) mechanical relay contacts; they will be much less tolerant of circuits that have different voltages or currents at different times since their response will not be linear as an electronic device.
Finally, chaining lots of SSRs is harder due to the voltage drops that exist across electronic switches. Although these could be designed away by using a specially designed circuit and high-impedance buffer at the end, they would not be simple drop-in replacements for existing relay circuits where there could be 20 relays in circuit between input and output.
As soon as you start having to design things differently for SSRs, their value decreases because in most cases, you would instead use a microcontroller and directly operate high power relays with the logic embedded in software to avoid the high number of external switches and points of failure - that's what Solid State Interlocking does in railway signalling systems and makes for much less equipment to manage than traditional mechanical relay interlocking.