Translation:Trains are big.
Depends on perspective. Compared to the universe they're tiny. There are also different kinds of trains. Where I live there are often trains which are quite small. (The GTW 2/6 Spurt (Not the 4 part train, but the 3 part train (including the engine))). Most trains are big though.
They are not the same thing.
Big train - The adjective is directly modifying the noun. Whereas in the sentence
Big is a statement/description about the train.
So, having Big Train in the answer is wrong, since we're not directly modifying train, just saying that it is big.
たち is not a true plural, it only ever means "--and their associates". For example, 私たち can be translated as "we/us", but it most accurately transliterates as "me and my associates". This is why you can only use it for people; you can't attach it to inanimate objects like trains and expect it to work.
Not sure where to post this, but I found it interesting, so I thought that I would share. I was trying to remember how to say a train in general; although I lived in Japan for a bit, I am not a native speaker, so take the following with a grain of salt. To say a train generally, one can say, 「列車(ﾚｯｼｬ)」。
電車 is electric train;
汽車 (ｷｼｬ) steam train;
列車 (ﾚｯｼｬ) train/ railway train