Locomotive Questions

I have 2 in-cab DVD's. One is a cab ride from Washington Union Station to 30th Street Station in an AEM-7, the other is a cab ride in a GG-1 from South Amboy to Penn Station.

Every once in a while I hear compressed air discharging in the cab. Is this normal? If it is normal, what is the air for?

Another question, and somewhat personal. What do engineers do if they have to use the restroom? That may sound silly to some, but I'm a non-railroader and I don't know these things.

Yes it is normal. Air blowing in the cab is on an AEM7 was from the release of the independent brake. The automatic brake application/release sound would come from the brake rack in the machine room, but you could hear that sound though the bulkhead wall if you were running from the R end (which is where the brake rack is located.) AEM7's had chemical retention toilets located next to the air compressor.

What is an independent brake? Is that different from the brake system?

The independent brake operates the brakes on the loco only,
while the automatic brake controls loco and train brakes.