So, as we all know, this past Tuesday, a man known as Frank James opened fire on a packed rush hour service headed for Manhattan. He has since been arrested. 
I feel like it was only a matter of time until a mass shooting on a subway train in the city was going to occur, I just didn't know when. Now that we've seen the outcome of this horrendous attack, here are some proposals I came up with to make the subway safer. You can comment on my proposals and give constructive criticism if you wish.
Dog sniffing and more frequent cargo searches by NYPD must be conducted at stations. If someone has a lethal weapon on them and are carrying it illegally, the NYPD can carry out a bag search and possibly nab a suspect. Dog sniffers could probably do the same, although I know that they're mainly trained to find drugs.
Sensors at turnstiles could also be a thing, but this may be quite expensive to install in a system with almost 500 subway stations. If the TA got enough funding, they would be able to install these sensors at turnstiles, gates and revolving doors.

For now, that's all I have. Please comment below on what you think about my ideas!