Hi There is nothing wrong with having a single track circuit . Whilst running two trains at the same time could be good a single line with passing loops can offer operating interest as well. All though an 8x4 board can accommodate a double track there is a risk it could be come all track. It really depends on your preference and want you want from your layout. One thing to consider is that at 8x4 before your longest train may only have four coaches any more and they may be classed as tail chasers . For example a while back I took some measurements and a Castle class and four Collett 60ft coaches work out at around 4ft. Its just a thought to ponder on . Jim