Cart noodle is a kind of à la carte noodle which became popular in Hong Kong in the 1950s through independent street vendors operating on roadsides and in public housing estates in low-income districts, using carts. Many street vendors have vanished but the name and style of noodle endures as a cultural icon.
With many immigrants arriving from mainland China during the 1950s, hawkers would sell food out of a cart roaming the streets. Some vendors specialising in cooked noodles would sell them with an assortment of toppings and styles. 
Historically, the cart frames were assembled out of wood with metallic basins. It allowed the heat inside to cook the ingredients. In the old days, it was possible to receive large quantities for a cheap price. The noodles were considered "cheap and nasty". Cost was generally low to appeal to the average citizens. It was known for its poor hygiene. As such, they were also commonly referred to as "filthy noodle" (嗱喳麵). Since hygiene standards rose, many street vendors (licensed or otherwise) have vanished.
The name and style of the noodle endures, and remain widely available in low- to mid-end eateries. The price may vary depending on the combination of ingredients, or type of establishments. On the contrary, because the noodle is now defined by its retro style, it can sometimes be found in higher-end establishments. It may offer costlier types of ingredients at a higher price.