From the activities of the Budapest maintenance workshops, before the World War II
From the beginning of the century, the two major tram companies in Budapest founded maintenance yards from unused storage halls. The larger company (Budapest Street Railway Co. - BKVT) had its workshop on the Buda side of the town (right bank of the river Danube) - later after the unification of the tram companies become the Buda-worshop; the smaller company (Budapest Electric City Railways - BVVV) made his on the Pest side (left bank of the Danube), and it became the Pest-workshop. The other small companies used either the workshops of the greater companies, or used their owh storage halls for repairments. After the unification in 1923, the Budapest Capitol Transport Corporation (BSzKRt) owned very colorful rolling stock: from motored cars had around 25 types, from trailers around 20. That is the reason, why the maintenance yards in these years were mainly trying to make uniform parts for the different types to ease the mending of the cars. In the '30s the yard started a program to reconstruct the older cars eg. mending the old motors to double their power, installing current-recuperation, making multiple units.



Multiple units: for experimental reasons some cars were rebulilt as so-called Monterose-muliple units. The original cars could not form longer trains with more than one motored cars, because the driving commands could not be transferred to the other cars. With the installing the Monterose-system, near the controller was a servo motor, wich could also drive the car. The rear car compared the current its motors with the front car's, and if there was a difference, the servo gave more power for its car. The problem was with this system, that the rear car was accelerating always a bit later than the front car - that caused twitching in the train.

This is how they rebuilt the BVVV's M type tram in 1931. (3006+5214+3007) Later they changed their form back to solo cars. 
Building twin-cars: some tram routes of the BSzKRt ended in a terminus without turning circle. That caused sophisticated coupling there for the trailered units: to avoid that the company built from two solo cars a twin unit. They removed one of the two motors from both solo cars, and made a two-car unit with the electrical equipment of a solo car with driver cab on both side: so they didn't have to change the old electric equipment. This rebuilding was the first of its kind in Europe.
The BVVV's D type car had originally one motor - so by forming twin unit didn't change the accelerating power of the cars. On the second picture there is a twin unit formed from BLVV's (Budapest-Szentlõrinc Local Electric Railway) solo cars and trailers. Alltogether before the W. W. II. was formed 59 such units, but after 1945, they made much more (around 180 pairs). 
Articulated units: they built from the BLVV's above mentioned twin-cars an experimental articulated unit (BLVV 25-26, nicknamed "Adél"). The car become four motors, and had a multiple-unit device, because a servo in the rear unit copied the movements of the driver's controller. The second unit with other type of multiple-unit equipment were BKVT's Q type cars (1393-1394, nicknamed "Gizi"). There was a third car too, from the Q type (1402-1403, nicknamed "Zsuzsi").
Making trailers: some of the early types were rebuilt as trailers.

For example the BVVV's C type cars from 1896 were used as trailers until the '50s. On the second picture, we can see it at a busy cross on the Budapest Great-Ring. Notice the cars original Dresdener-underframe - later it was rebuilt with wider axis distance radial truck. 
Second entarnce lanes: For shortening the stay at the stops on one motored car (BVVV K type)and more than 100 trailers was built second entrance. On the picture we can see the BVVV M type trailer. Although after the W. W. II. the trams pulled mainly trailers with doubled entrance, the motored cars stayed with single entrance - so still remained the loss of time at stops.

Summer-cars' rebuliding: at the beginning of the century there were open cars in use for the summer season. Because of the growing crowd on the trams, the BSzKRt rebulilt these cars with solid walls and glass-windows, and used them in the full year.
The first car is BKVT's H3 type (on the second picture is the car with three windows), the trailer is BKVT's R4 type. Between the Miskolc pictures we can find the R4 trailer, but the H3 too rebuilt as a trailer
Current-recuperation device: on the trams we can brake with the motors using them as generators (rheostatic brake). The current produced by the braking if it's led through the pole, could be used for accelerating other cars. The BSzKRt supplied around 50 cars with such device: but it needed also a battery (in the lower extra box) and also a relay, wich watched the voltage (in the upper extra box) wether the pole is suitable for recuperation. After the W. W. II. the device was taken down, because of mending problems.

The first picture is a BVVV G type car without recuperation, the second is a BVVV F type with recuperation device. These cars were almost similar electrically before the rebuilding. 
Producing new trams: during the World War II. the bus maintenance workshop didn't have enough work, because the buses were used by the military. That's why they started to produce new tramcars - the F1A type. The car had extremly good accelerating power (2x92 HP) - the today working cars have similar power. Alltogether they made 55 cars.

[After World War II.]