The Debian package that James has kindly provided on Bridgewater-Brunel specifies a dependency on an older version of libpng. This version of the library is not available in recent versions of Ubuntu (https://packages.ubuntu.com/search?keywords=libpng12), and I imagine other Debian distributions too.
Steps to reproduce:
1. Run Ubuntu 16.04 or more recent.
2. Download the latest Debian package (http://bridgewater-brunel.me.uk/downloads/nightly/packages/simutrans-ex_12.9000.14.deb).
3. Attempt to install using dpkg.
The latest version of the required package is libpng16-16 (https://packages.ubuntu.com/search?keywords=libpng16) and I was able to install and run Extended on Ubuntu 18.04 by changing the "depends" line to "libpng16-16" (and making other changes to be reported in separate bug reports).