Some of the original Gilbert parts breakdowns indicate that the original cap was a Planet PA12A820. I can't find a cross-reference for that particular item, but Doug Peck at Portlines.COM offers a replacement:
[SKU ID:2625] Capacitor for speakers 100 mfd (replaces Planet PA12A820) - $2.00
These are listed for diesels and steamers. I haven't opened up any of my locomotives to check, but Doug is usually spot-on when it comes to replacement parts. You can always call or email Doug for specifics if you have further questions. Hope this helps.