FUSE RATING QUESTION

It's the first time I heard of a fuse inside a locomotive, but my guess is the fuse has to be at least 3-4 amps if it is connected between the third rail pickup and the motor, e-unit, and headlight.  Most postwar single motor locomotives with a headlight, smoke and whistle consume 35-40 watts of power when running at full speed (18 volts and roughly 2.5 amps).

Most parts sources offer this item for about a dollar, and my guess is it could be likely bypassed if needed. MPC/LTI locomotives do not have circuit boards (save for units with electronic e-units and/or horns or railsounds), and as such they are more resistant to overloads. Modern locomotives are essentially computers on wheels, so the circuit breakers on modern transformers are much more sensitive to prevent boards from getting fried.

Hope this helps.

-John

If you want some protection, consider a 2 or 3 amp PTC instead of the fuse. 

What is PTC?  I am installing a new smoke unit after the old one fried.  The fuse had blown and the loco is so old it is hard to read the writing on the fuse, hence my query here. 

The PTC is a resettable fuse, it protects the circuit and opens with excessive current, then automatically restores when the excess current is removed.

Here's one that holds at 2.5A and trips at 5.0 amp.  http://www.digikey.com/product...R/F1982CT-ND/2690450

These are good for protecting wiring that might get across the rails.

The breakers on modern transformers are there to protect the transformer and wiring, and will not by design prevent boards from getting fried.  The damage is done long before any fuse blows or breaker trips, and is cumulative(which might make it look like the breaker is saving your expensive loco - for a while). 

To protect sensitive electronic circuitry, you really need transient voltage suppression.