An off-the-shelf vehicle is likely to look like one of these (the most recent models from the main manufacturers). All of these models are widely used worldwide; I've noted which ones are used in the US and Canada. All images from Wikimedia Commons - click for the file information page. In many cases I would expect to see a blunter nose for the Type 10 - long streamlined noses will not fit in the restricted loading gauge of the Tremont Street Subway.
Siemens S70/S700: widely used in the US (Portland pictured)
Bombardier Flexity Freedom: used on several Canadian lines (ION pictured)
CAF Urbos: used on several US lines (Houston pictured). The Type 9 is a modified version of the Houston model.
Alstom Citadis Spirit: used on several Canadian lines (Ottawa pictured)
Kinki Sharyo: a Type 10 would likely be similar to this P3010 in LA.
AnsaldoBreda (now Hitachi Rail Italy) Sirio: not yet used in North America, but common elsewhere (Milan pictured). Their last US effort, to my knowledge, was the
P2550 for LA in 2007-2011.