I haven't a clue, but if you were to ask your question in a suitable place, e.g. the DCC Sound Forum, you're sure to find someone who knows. I'm surprised you haven't yet gathered that blogs are not the right place for questions.

Nick

I agree with Buffalo that blogs are not meant for questions, since there are many specialist forums intended for exactly this purpose.

However, I happened to be reading about this topic in the book "How Steam Lomototives Really Work" by Semmens & Goldfinch, OUP 2000.  I quote:

"One of the authors had the opprtunity to ride on the LNER Garratt on the Worsborough Bank when the combined exhaust beats were obvious .... until one of the engines slipped.  After the driver had obtained full adhesion, the two ends were very much out of synchronisation, with a jumble of twelve beats per revolution.  However, the two exhausts immediately began to converge, and within a very few yards the two engines were synchronised again, which was maintained until the top of the bank." 

The book goes on to suggest that this is a resonance phenomenon and that the forces involved are sufficient to cause micro-slipping until the engines are in phase.

Not everyone agrees about this - try reading the thread in the forums at http://www.rmweb.co.uk/community/index.php?/topic/42598-Heljan-beyer-garratt/page-32 and onwards.

Thankyou Nick and Mike for your help,i have put on the dcc forum,sounds like the Olivas may be correct.