In most subway systems, trains outside of peak hours are not full-length but rather fill only part of the station. In the subway systems I'm familiar with in the United States, short trains stop in the center of the platform. But in Lisbon and Madrid, short trains pull forward as far as possible before stopping. Therefore, you should stand on the forward end of the platform. But where is the forward end?
The forward end is the end that has the television monitors or mirrors, because the driver of the train uses them to check that the train doors are clear before pulling away.
Other tips: The vending machines in the Metro stations do not sell multi-day passes. To buy a multi-day pass, go to "the man in the little house", which is the cute phrase a hotel employee used to describe the ticket agent booth. The ticket booth is not manned at night, so plan accordingly.
(Just in case it wasn't clear: I'm not mocking the employee's English. After all, his English was far, far better than my Portuguese! Besides, the man in the little house is probably the phrase I would have used if I had to describe the same thing in Swedish, German, or Chinese.)
You don't need to take your pass out of your wallet or purse. Do like the locals: Place your wallet or purse directly on the sensor (oriented so that the pass is close to the sensor, of course) and hold still. After about one second, the sensor will recognize your pass and open the gate. A common mistake is to rub the pass in circles against the sensor. In my experience, moving the pass around just makes it harder for the sensor to find it.
Your multi-day pass covers all municipal subways, trams, buses, and elevators.