The MSTS 1 Route Editor can be a beast to use, but it's capable of some amazing results, here you can talk with the wizards that are building some of the fantastic routes available and learn how to make your own.
By complete coincidence I downloaded it the other day, and its quite good. I'm doing a set of activities for some steam excursions using a J15, B1 and posibly another using a Brit to celebrate XXX years of it being open.
In theory, you should not need to Unstore the defaults to install a route, but I always play safe and use Maintenance mode with 'include defaults' checked.
Jonathan, you're going to need to install all the default routes if you want to add more routes, and probably use TrainStore to regain your short loading times.
BarryH - thenudehamster
(nothing to do with unclothed pet rodents -- it's just where I used to live)
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Any opinion expressed above is herein warranted to be worth exactly what you paid for it.
I've just downloaded this route, and I wanted to let you know I think it's great. A friend of mine's garden backs onto the line just west of Mistley so I know it well, and it is uncanny! Thanks for all the effort!