Brushes. Although round, I suspect (but don't know) that the Graham Farish N gauge brushes and springs (Part No 0129) may be adapted to fit. Update - Denm contacted me via the Visitors Log Book and confirmed that the Farish bushes do indeed slot into place without the need for any modification. The picture on the left is linked from http://www.lendonsmodelshop.co.uk - who list this part and also list some spare bits for Minitrix.