Hopping freight trains was indeed once quite common in the US though the security clampdown following 9/11 has made it much harder to do. But there are some significant differences between the US and here when it comes to freight hopping.
Firstly our railways are largely fenced off from the outside environment making access to stationary freight trains very difficult unlike the US where fences are unusual and avoiding the railway police patrols is the biggest obstacle: as suggested above since 9/11 such patrols are much more enthusiastic than used to be the case. Secondly many US freightcars are relatively rideable, ie climbing on/off them from/to ground level is often facilitated by suitable steps/handrails (provided for the convenience of yard staff) and the vehicles themselves may well have a place where it is possible to ride with a degree of shelter, if not real comfort, while also being able to take in the passing views. Very few such vehicles exist here. Thirdly the US is a big country where railway lines spend a lot of time away from human activity, not to mention a lack of passenger stations, meaning unofficial riders have a reasonable possibility of avoiding detection. That's definitely not the case here; even if you could find a way to get on a freight without injuring/maiming/killing yourself you would likely spend most of the journey trying to stay hidden from view given the number of stations or adjacent roads on the network: it would not be any fun!
On a visit to the US some 20 or so years ago I was able to purchase a book entitled "Hopping Freight Trains In America" which provided a great deal of insight into the dos and don'ts of the activity. But it also made it clear that attempting it here is largely pointless, the conditions are just too unfavourable.
