The lead <a href="https://en.wikipedia.org/wiki/pony_truck" class="extiw" title="w:pony truck">pony truck</a> of "Southern Maid". This bogie is free to move from side to side to allow the loco to take curves. However this movement is constrained by two large side control springs and it is they that literally pull the loco into the curve.
<a rel="nofollow" class="external text" href="https://www.flickr.com/photos/elsie/">Les Chatfield</a>
CC BY 2.0
<a rel="nofollow" class="external text" href="https://www.flickr.com/photos/elsie/477215565/">Guider wheel</a>