Mark, assuming that you are asking about the capacitor-based circuitry, here is a passage I recently posted in another (ESU-specfic) thread in the DCC section of the forum, but it pretty much applies to any brand of decoder.
ESU decoders have "token" capacitors on-board. Couple hundred microfarads - just enough to coast uninterrupted through some very small power drops (few milliseconds in duration). Even the additional caps added by modelers (usually less than a thousand microfarads) offer only slightly longer ability to coast through power drops (maybe tens of milliseconds). These are more to keep the decoder's microprocessor (brain) which runs all the decoder's functions (including sound) from re-initializing.
True Keep-Alive circuits which use SuperCaps have capacitance in few hundred thousand microfarads. Those will power the model for a second or two without any power coming through the rails. Those devices are too large to fit in most N scale diesels, but sometimes are being installed in steam engine tenders (where there more available space).
