Fiddling with electronic devices while riding Metro is generally a bad idea — especially if you’d like to leave the transit system still in possession of said devices. But Deborah Forbes is grateful she had her Kindle last Friday.

Inspired by my column that morning about folks illegally drinking coffee on the train, she handed a copy of Express to a fellow Orange Line passenger who was sipping from a steaming cup and asked her to read it. That’s when things got ugly.

“She threw the paper on the floor, and told me I should be careful who I [messed] with because they might be ‘carrying,’” Forbes says. She ignored the threat and pulled out her Kindle. Smart move, because the coffee drinker then threw the cup at her. Using the device as a paddle, Forbes batted the projectile away.

There are two lessons here. 1. Try not to confront unstable people on Metro. 2. There’s a lot more you can do with an electronic gadget besides organizing your Pinterest board.

The latest reason to risk the safety of your iPhone or iPad is an app released this week. Metro Master ($2.99), created by two recent GW alums, tells you the optimal place to board so you’ll be situated as close as possible to the station exit when the train arrives at your destination.

“I love being the first one on the escalator,” says David Glidden, who realized that making sure he could manage that at every station would require research. He started taking notes on his iPod touch to remind himself of which car to get on depending on where he was headed. The process became such a pain, he enlisted his pal Andrew Thal to help create a program devoted to exit strategies.

Now, it’s a cinch to find out that if you’re on the Red Line and want to leave Farragut North via the east L Street exit, you’ll want to make sure you’re at the first door of the first car. If you’d rather go for the west L Street exit, get to the third door of the fourth car.

Don’t care how far you have to walk, as long as you have a seat? The app lets you check rider usage data to determine how crowded each car generally is, so you can aim for one with fewer people. But I’m not promising they won’t be unstable.