Howdy,
If this is the only means to shutdown the engine on overspeed, did you want to consider a dedicated overspeed switch?
(I would be nervous wrt using a prox-probe direct into some low-cost PLC as my only protection against overspeed.)
If this is a loco engine, we are talking about a couple thousand hp, correct? If this engine is driving an alternator, can you not get the engine rpm from the alternator frequency?

Lots of companies make these switches (see link below for Woodward type). The Woodward unit is nice since it has the environmental ratings found around an engine (ie temperature, vibration, shock, etc)

http://www.woodward.com/speedswitches.aspx

Yeah, I'd not want to use a PLC input for an overspeed event either.

Using AC frequency isn't always a good idea either, as you can conceivably have an overspeed event unable to be detected if the voltage goes below the detection threshold (i.e. loss of excitation and subsequent loss of load causing overspeed as an example). It also doesn't work for detection of engine at cranking speed, if the excitation is disabled below a certain frequency.

You can get discrete over / under speed detection relays, I'm not a fan of them but they're more effective than relying on a PLC. My preference is to feed the magnetic pickup into a dedicated generator set controller (Deif, Woodward, Comap etc) and use that for your speed protection, along with everything else the controller does. Crompton make some discrete relays that are suitable for speed protection if a cheaper option is desired.

Thanks for helpful info.we drive a DC generator so ac signal will work.we also use a aux. Generator to charge the batteries and Excitor for generator. The power for safety circuit comes from batteries and a separate supply driving a relay which comes directly from the aux.generator. take it this is for a stall of engine but not sure if this will protect against loss of signal from proxy .we have a Woodward governor but all it has a fuel solenoid. Shutdown is by removing power to this solenoid.The protection we use is bought from cat no 100_5675_4 but could be built by "Woodward"