I guess you have heard all about George Bennie's Railplane in the 1930s?
The idea was that passengers would travel in the overhead fast monorail system whilst much slower goods trains would continue to use the lines underneath.
An evolutionary "Dead-end." Aeroplanes are much better at this sort of thing.
See this article from the Daily Mail Newspaper in 2012.
http://www.dailymail.co.uk/sciencete...penniless.html
Is it just coincidence that this thread has been put in the railway modelling thread?!