I'm thinking he might need a bigger brake pad there, huh? It's only about 100-times bigger than those on my bicycle.
Seriously, I suspect the big brake pad is probably needed. Those wheels looks quite heavy, and with much of the weight in the rims, they probably have quite a bit of inertia when the cart gets rolling along at some pace.
There is a company in Franklin, NH that makes a neat version of these and sells parts too. Railriders. www.railriders.net. They have photos on their website, (I have no connection with them so this isn't a sales pitch).