It can be hard to get the cab these days thanks to forums, I also think the Travel Channel played a part in this
Each train (on all lines) does have an assigned station, but that's not for the purpose of the FCE (Front Cab Experiences), at least not anymore. On MK Express and Epcot, the guests must exit at the next stop. On MK Resorts, guests can go to any of the 4 other stops on the loop. A full loop is typically not permitted on any beam. On the MK Resort loop, the CM will check to see if the cab is empty or if the guests are leaving.
At night, sometimes the MK Express goes one-way to MK. If loading the Resort trains from the Express side, then the front cab is not typically accessible. Drivers have allowed guests from that side, but it's bad practice because the loading CM may have denied requests from guests on the Express side. Then, the CM gets a very very livid guest after watching people get into the cab.
There isn't a special bench anymore. Some cages have a small queue area, but standing there means nothing without asking the CM. Some CMs don't even load from that gate, and instead load from the CM access gate. Four people is the MAXIMUM. Babies on a lap count as a person. I sometimes have guests tell me that their toddler is "half a person." Wow! That poor kid.
In rain and low visibility, cabs are not loaded. This is to decrease driver distractions. The other cases are when maintenance is aboard and during the first few days of drive training.
Aye, just be courteous about it. Don't be upset if the driver doesn't really talk much, safe operation of the trains is priority one. Some drivers are not good with handling many things at once, such as carrying a conversation when there is a lot of important radio traffic, keeping visual wherever possible, watching the MBS, system malfunctions (minor malfunctions are quite common), and constantly fighting the speed system in order to lower the cycle time. The priority is train operation -> radio -> guests in the cars (such as making spiels) -> guests in the cab.
If you have loud kids, please refrain from taking them in there. #1 cause of an overrun is due to driver and guest interaction. 3 overruns and the driver is gone.
Also, avoid saying: "don't these drive themselves?" No. It can't be automated without demo'ing and replacing all the beams with evac walkways, stairs, and railings. Of course, also replacing the 12 trains and installing a system to track the trains. Not many 12-train automated systems out there. The NY Metro is powered by drivers... so are we. The Tokyo Yamanote line has drivers, so does WDW.