I hope you take power to your accessories from the track, not the trafo, since they both are grounded through the track. But this limits the power that can be used by the trains, since the accessories use digital power. The 18 VA trafo is just enough to power the MS anyway, so it is likely that the trafo limits the capacity in your case, not the MS.

There is an easy solution if you have a second trafo available: Disconnect the accessories from the track and feed them through their own trafo. This trafo will have to use "common ground" with the track, so it can't be the same trafo that feeds the MS. This will reduce the stress on the MS and give you more headroom. If you have a 32VA trafo that can be used to drive the MS it is even better, since you get maximum available power from the MS.