Scott's CE dic. "loadings on bridges for the purpose of the design of the structure are laid down by authorities such as AFNOR, ASTM, BSI, DIN, often by agreement. Railway bridge loadings (RU loading, RL loading) include allowances for centrifugal force, luching, noise, etc.

Well, you already know what it is, but here is a ref in Spanish which helped me to get the answer:

CARGA VIVA MOVIL EN PUENTES

El código AASHTO define diversos tipos de cargas móviles que actúan sobre los diferentes componentes de los puentes: camiones de 2 ejes (HS20, HS15), camiones de 3 ejes (HS20-44) y cargas distribuidas equivalentes al flujo vehicular, con eje de cargas concentradas.

Mientras los camiones de carga idealizados simulan el efecto de la presencia de vehículos sumamente pesados de 2 y 3 ejes, la carga distribuida equivalente con eje de cargas concentradas simula el efecto de un congestionamiento vehicular sobre el puente. En ambos tipos de carga se presupone que actúan sobre 1 carril del puente con un ancho de 10 pies (3.05 m).

Next there is a picture of the camión or tren de cargas de 2 ejes.
http://www.espe.edu.ec/cursos-e/civil/puentes/puentes05.htm

This mentions the AASHTO code, which the dictionary doesn't, leading onto double checking on English pags what I found in the CE dic.

Design of continuous slab/girder bridge. Including AASHTO Bridge loadings.
http://bridge.ecn.purdue.edu/~ramirez/ce672.html
More in the PDF ref.

This loading consists of tracked vehicle, the loading of which is 70 tonnes and a wheeled vehicle of 100 tonnes loading. Both the vehicle types have specified dimensions which are to be observed during the live load analysis in bridge design.The choice from the two types is made depending upon the anticipated types of vehicles to travel on the bridge. Bridges designed for Class AA loading should be checked for class A loading also. As under certain conditions heavier stress may be obtained under class A loading.
http://www.iitd.ac.in/cgi-bin/nph-p/http/10.116.2.57/course/...

This ref. is in Spanish tren de cargas using the German DIN code:
http://www.ingenieria.read.net/guayabo.htm



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Note added at 2002-05-21 17:17:14 (GMT)
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Another ref:
Comparison of Bridge Loading of existing goods vehicles and 44 tonne vehicle.
Bridges must be able to carry permitted vehicles with adequate safety margins. This means that bridges on motorways and most trunk roads should be capable of carrying the heavier \"EC\" vehicles to be allowed on our roads from 1 January 1999. Limiting axle weights and spacings and spreading loads over a number of axles are as important for bridge safety, particularly short span bridges, as they are for minimising road wear. Gross weight is more relevant for longer span bridges, where the total potential weight on the bridge in a traffic jam is an important consideration. The greatest loading for a given length of vehicle is produced by certain types of 32 tonne vehicles already in use. A number of these nose to tail on a bridge would produce a higher total loading than the fewer 44 tonne vehicles that would occupy the same distance

As can be seen on this diagram, the loading on this bridge from existing vehicles at 192 tonnes is far higher than the loading from the 44 tonne vehicles at 176 tonnes.
http://www.ilpga.com/poc_gvw.html

>From a bridge loading perspective, the trend towards multiple-trailer vehicles is very important for bridges with spans in excess of 15 to 20 m. The grouping of trailers together demands a rethink of the lane loading and multiple lane reduction factor provisions of the bridge design code.
http://www-mech.eng.cam.ac.uk/trg/vrnet/1997/trans50.html