Now, to make a new train, you'll need two reference images: one for the front and one for the side.
Both must be absolutely horizontal and if you want to use them as textures, they must have been taken in neutral light condition (e.g. absolutely not in backlight conditions).
Wikipedia almost always lists the dimension for lenght and width (and sometimes height too). Mind that it's better to go to the "native tongue" pages for the train you will make (in this case, use the japanese wikipedia version.
https://ja.wikipedia.org/wiki/新幹線200系電車
"Kakeyama photo studio" (
kakeyama.image.coocan.jp) has perfectly horizontal side scans of most japanese trains, and will be instrumental.
http://kakeyama.fan.coocan.jp/tec/200h.htm
If you can't find perfectly horizontal front pictures, "trainsface.art" (
trainsface.art.coocan.jp) comes in your help. Note that you can use these also for intermediate cars, as they have the same loading gauge as the cab ones.
http://trainsface.art.coocan.jp/site2005/railway/jr_jnr/jr_top_shinkansen.html
Now, learning to make a train in sketchup is a lenghty process, wich i won't be able to explain in enough detail on this forum.
After i've finished doing all of the Tokyu 1000 Series variants (i'm almost there) i'll start making the Tokyu 8090 and 8590 Series trains (also in sketchup), so how about this: i'll record the making process so you can follow step-by-step what you have to do.
Just be patient a couple, maybe three weeks and then you'll see.