This is a cute apartment a block away from the Colosseum. If you're able to walk and your luggage isn't heavy, you can easily walk from the train station to the apartment. Andrea gives very accurate directions.
There is a lift in the building, but it is very tiny. If you're wearing a backpack, you won't be able to turn around while you're inside. (Very European!) The living room/dining area are open and makes the apartment feel spacious. The bedroom is very minimalist and the bed is very comfortable. There was even a pack n play for our son.
Be warned, the bathroom and kitchen are small. I am 6'4" and my wife is 6' and there is no leg room when you use the toilet because it faces the shower. The shower is also tiny! But if we could make it work, anyone can. The kitchen is small, but it has everything you need. I'm assuming you'll be enjoying the local food rather than cooking anyway.
There is a little 2-person metal balcony with table and chairs. It only has a view of the back alley, but was a nice spot for us to enjoy a cup of espresso when we woke up. It's pleasant to have the windows open during nice weather, but the street noise can be a bit much. The windows are thick, so when we shut them, we couldn't hear any of the street noise.
If you open the window and look down the block, you can see the Colosseum (there are some pictures that show the view). It is close enough that we were easily able to wake up and walk to the Colosseum, with enough time to stop and get a pastry and coffee on the way and still be 2nd in line when it opened.
It's in a great location and a fun little LGBT friendly neighborhood. Lots of cafes, tapas places, wine bars, and restaurants in the same block. There is also a pharmacy if you forget anything or eat something that doesn't agree with you half a block away.
We used the hop-on-hop off bus to get around (and to get some commentary on the sights around the city). There is a booth where you can get tickets right at the end of the street (towards the Colosseum) if that is something you're interested in and you can get tickets for some of the sights/ tours as well. We picked up Vatican tour tickets from here.
It was a perfect place for me and my wife and our infant son to stay for our long weekend in Rome. If you're good with taking turns in the bathroom, this apartment could house 4 guests or possibly a family of 5 comfortably.