Did you read Greg Neff's posting and his friendly warnings ? In fact I kind of guessed as much when you said about TCN.

I have tried to work out ( unsuccesfully )a system for speed logging of locomotives with a time stamp in a NV RAM. This cartridge was then supposed to be pulled out and inserted in another base unit for permanent archive of the information for post journey analysis or in case of a mishap. Not a complex job - The logic / implemntation / checkout - all these were fine in the lab. But once this stage was crossed implementing it in real time in a real locomotive threw up so many issues in terms of ruggedness, noise immunity ( ever seen those 25kV arcs on the overhead lines when the locomotive is bolting at 150 kmph +?), reliabilty etc.

I dropped the project and lived happily ever after.

Raghu