Complex sentence: This was problematic in that the new design was so large that it only just conformed to the maximum loading gauge for the main line and it was sufficiently heavy that it was close to the Civil Engineer's maximum weight limit.
Split into simpler sentences: This was problematic in that the new design was so large that it only just conformed to the maximum loading gauge for the main line. Moreover, it was sufficiently heavy that it was close to the Civil Engineer's maximum weight limit.