Measurements from prints/pictures
If the blueprints you have show the wheel sets you should have a dimension (3') and from that you can extrapolate the dimensions or find the scale, provided the plans are to scale. I'm guessing that you have a photo-sated or photocopied set of plans that are reduced in size and is illegible or missing the dimensional data. If you have a full copy of the plans the scale should be mentioned in the information boxes which should be at the bottom of the page (as it is read). My suggestion is to get in contact with a local drafting (cad/cam?) instructor and trade information, like a copy of the plans for the dimensions (or the process to get them). A professional draftsman (draughtsperson) might also help you but it may cost you more than time. Another place to look is your local senior center as it is full of people with a lot of information and a lot of time, perhaps one of them was a draftsman or an engineer.