Your Penn Line k4 is most likely powered with a Pittman Motor. One motor brush may be connected to the motor frame with a brass strap and in turn to the loco frame. This is a no-no for DCC operation.
Based on your user name, is this locomotive O scale or HO ?
If HO, the first thing you need to do is get an Amp meter and measure the motor's current draw under a heavy load. This can most easily be done by connecting the meter in series to one of the wires from a DC power pack to the layout and run the locomotive while it is pulling a long string of cars; so many that the wheels are almost spinning. If the motor draws more than 1 Amp, you will probably need to remotor the loco using a can motor because decoders that can handle more than one Amp continuous are very expensive and hard to find.
I have an old Penn Line K4, and I was wondering if it was even possible for a decoder to be installed with the amperage these dinosaurs suck. If so what kind of decoder?
Thanks!