[lucb1e]

So you raise a fence around it (stored on the sides, underground of course), or slide a roof in (once again, stored below the surface) to prevent anyone from going in even if they wanted to. This doesn't sound like a biggie to me.

[ceejayoz]

The ongoing maintenance of tens of thousands of sliding doors, elevators, and retracting fences sounds like a biggie to me.

[Robotbeat]

Every subway contains a whole bunch of sliding doors that open and close every single stop.

[ceejayoz]

Yes, and they frequently need repair and maintenance.

Now imagine having tens of thousands of subway cars instead of a couple hundred.

[cma]

There are tens of thousands of teslas out there already with automatic doors.

[Robotbeat]

And (most?) new minivans nowadays. And most cars have power windows as well. And most supermarkets have sliding doors as well. And probably most garages in the suburbs have automatic doors. This is actually run of the mill technology.