Colin, it is possible, but only if one understands and accepts that there will not be shadow detail comparable to a higher speed film (like D3200, with a true ISO of about 1000) at 1600 or a 400 speed film at 400. In many respects all one gets are better separated and printable mid tones and highlights because the shadows just aint there....
This might all sound pedantic, but lots of people can't understand why they do not get prints like someone else's when they both push the film to, say, 1600. Its usually that one scene needs shadow detail and the other has none or can stand to lose it. The former will fail miserably as the shadow detail is missing at 1600. The other looks great because it was never there in the scene in the first place, or if it was, it did not matter.
So yes, you can push 400 films to 1600, but at no point does the film perform like a 1600 speed film and you always see the low values drop down the scale or off the bottom end entirely. I think its really important for people new to pushing to understand this otherwise they wont be able to make sense of the results they get and know when its a horribly bad idea to 'push' films in this way. You can rate TriX at whatever fictional speed you wish, but it will never come close to a true 1600 in the shadows. This does not mean the result wont look great...
PS try Tmax 3200 at 1200 or so in Xtol 1+1 at 20 degs C. I printed a 20x16 last week and the grain is miniscule. Absolutely tiny, and the other six rolls look exactly the same. It actually looks finer grained and tighter than unpushed HP5+. I was amazed, having used D3200 for years and won't be shooting any 35mm D3200 again in a hurry unless I want the grain. I find Tmax 3200 at 1000 gives me the same shadow detail as triX at 320 developed to similar contrast, so thats significant.
