Scale it from a known dimension?

from the web link there is a measurement of "just shy of 45 feet". 

The may be across coupler faces or maybe across frame?

the wheels are on other hand of a known standard size.

take the image into an editing tool (mine of choice is photoshop) and remove the perspective from the image so that you get the hopper square on. Pay attention to the 'roundness' of wheels to ensure accurate correction.

once you have it sure it's then a relatively simple case of scaling the image to your desired scale based on the factory measurement. 

Also manipulate the levels I order to bring out the detail you are needed

needless to say this doesn't get you the end views but may be enough to to get a 'good enough' representation of the hopper

All this becomes irrelevant though if someone finds a drawing or can supply accurate dimensions for you.

HTH

Simon