Personally, I've tried XTrkCad and not found it that helpful. I've also heard stories of people who've planned out layouts on it which then haven't managed to fit as planned into an area larger than the peramiters put into the computer. If you add that to the fact that you have to spend time learning the software properly to get its full potential, I would still go for a pencil, ruler, rubber and bit of paper. Obviously it depends on what you're actually doing, but I normally work with fictional minimum space termini and find the best way of designing them is as follows:
- Work out what you want include in your layout. This includes the basics, like whether you want the emphasis on freight or passenger trains, how many platforms you want, to a few ideals like maximum rake length and what kind of operations you want, to work out whether you'll need to include a goods loop or a bay platform or whatever.
- From this, start sketching a few ideas down. If you've got a good imagination this is relatively easy, if not, you can look for inspiration from a few prototypes from books, the web and even MSTS routes if you find something there that takes your fancy. Once you've got a few rough ideas down, you'll probably find a favourite fairly quickly - I always do. From here on, things take a bit more time and effort so it's worth checking your favourite design back to the original ideas you had to make sure you're not going to have to make any huge changes to the trackplan. You can mock them up in something like Trainz, which comes into it's own for this job, if it'll help but it's not essencial. The main thing to check for is the arrangement of points - at over £7 for your basic turnout, you want to make sure you have enough to keep your options open for operation without having any unnecessary ones that are just a waste of money really.
- The most time consuming stage of planning comes here - making your initial idea fit the available space. Unless you've got a large amount of space to play with or were particularly generous in stage one, some compromises will have to be made in order that you can still fit x number of coaches in the loop and managed to get that line from the bay platform back to the main line without the curve being too tight for some of your stock. Obviously, measurements are key here, and there's probably three main ways of tackling the task:
Undoubtably the best is to measure out the area you've got to work with on a large flat area (if you've already got the baseboards ready by this point, set them up), take a couple of spare points and relevant bits of stock and just see what fits where. Ideally you want a bit of leighway on all loops and headshunts to allow for longer bits of stock that you may later aquire and other things such as buffer stops.
Obviously, if it's your first model in that gauge or the layouts particularly large, the above is fairly unfeasible, so second choice would be to draw up some scale plans. Best to keep a calculator and a notepad out for this. There's a very useful pdf file
here with the dimensions for Peco OO, OO9 and N gauge turnouts which you can use to aid you and British N gauge is 1:148 scale (2mm/ft is slightly inaccurate as it turns out, giving 1:150.6 (in reality this minute innaccuracy will only make a difference if you're really tight for space), so, if you find the length of the longest prototype loco and coacg/wagon you're planning on running and divide it by 148 to get the scale length to use as a guide for train lengths, rake lengths and headshunts. It's best to give as much leighway as possible here to take into account the possiblity of slightly overscale stock and the couplings (I believe most prototypes are measured from buffer to buffer, yet model's couplings extend quite a distance if you scale it up, beyond this point).
Design software can be used for this process but, if you compare rubbing out and resiting a point on a scale drawing to accurately moving it on a computer and ensuring the rest of the plan is still connected properly and in shape, it will take a lot more time, hence why I don't recommend it.
With any of the above three methods, you may have to sacrifice a coach or wagon from your rakes or a siding or two, but you will end up with something you're happy with 99% of the time.
Apologies for the essay if you knew/have done most of the above or have your own method for dealing with these things. I write too much, I know...