It seems like the sound board may have a bad ground. Make sure all plugs are seated properly in their sockets. Remove the sound board and check all solder connections with a magnifying glass, insuring no bad solder joints. If all checks well, then the board may be bad.

Larry

The problem with decomposed foam insulation affected early electronic sound boards, especially in steam locomotive tenders. I do not know if there are any recent uses of that insulation in more modern trains. Most electronic boards are now mounted with standoffs, or in plastic carriers.
The El-Capitan locomotive is an FT diesel. The parts diagram http://www.lionel.com/CustomerService/ReplacementParts/index.cfm?doAction=productPartFilter&number=6-24568&productID=f39c0a1b-3d88-4b90-8292-84d73496a6a2&CFID=55610727&CFTOKEN=470be726b056f309-E0266715-0D3A-8FA3-D29F963113C5F1D6&jsessionid=8430d78ed46d236187016226405944215262 does not show how the board is mounted, however.
Check all connections between boards, and between the boards and the chassis with any ground wires, if used. Also check all solder connections on all boards as stated above. If everything checks out, then a board may be at fault.

Larry