T6B5 - the pinnacle of Tatra trams

Anatol Sorokin, translation by the author and Mike Russell

For many Soviet tram operators, the 1980s became a period of total domination by the Czech Tatra T3 tramcars above all other classes of rolling stock. In many cities, T3s were the only tramcars, whilst in others there were depots which specialized only in T3s. A discussion about the real or imaginary disadvantages of the T3 can be long and provocative but the "Tatrafication" of the USSR was an undisputable fact.

Amongst tramcars with an indirect resistor-based motor control system, however, in certain aspects the T3 is a unique design which requires a high level of servicing and maintenance. In the USSR there were serious difficulties with it: the introduction of the T3 was not easy, for unlike the good old MTV-82, the Tatra was unforgiving of servicing mistakes. But some older drivers told the author that the T3 is the most comfortable tramcar and if it was adjusted properly there are no problems with it. But if a T3 is not adjusted properly ... it is much better to drive an MTV-82, despite its uncomfortable cab.

Many operators considered the T3 as an ideal design, but the technical decisions specifications of the T3 had their roots in the 1930. In the mid-1980s, the T3 was produced over a period of 15 years without any major change in its design. But as time passed the disadvantages of the T3 became more and more obvious. The huge operational experience of the T3 in the USSR pointed the future way for tramcar development. In comparison with Soviet-built tramcars of types KTM-5 and RVZ-6, the T3 has the following disadvantages:

• small passenger capacity at full load;
• constant noise from the motor-generator unit;
• track corrugation and deformation arising from the single stage suspension bogies;
• uncomfortable passage to the front and rear doors.

Also, if not properly adjusted, the booster (electro-mechanical current control unit) went 'goating' - accelerating and decelerating the tramcar by kicks rather than smoothly.

The response of the Czech trambuilders to these drawbacks was the new tramcar T6B5, which is known in ex-USSR under the designation T3M.

The history of this tramcar began in the early 1970s, when the prototype car T5 was built. Two T5s were tested in the USSR (in the city of Kalinin, now Tver) in 1978, but the Soviet authorities considered the T5 as too expensive and no order for series production followed. In 1983, taking account of all remarks and comments from Soviet tramway workers, the Czech engineers built another prototype, designated as T6B5.

In the Tatra designation system, this means single-sided motor bogie high-floor tramcar with 7.5m distance between bogie centress and a 2.5m body width. In general, the term T6 designates a whole family of tramcars which differ in size, gauge and number of doors. Different versions of T6s work or worked in the Czech Republic, Slovakia, Germany and one car was even tested in the USA. The first prototype to be built was sent to Moscow for testing; the test results were very good and T6B5s undoubtefully were the most modern and high-tech tramcars in all Soviet Union but...

Moscow refused to buy T6B5s - these cars were deemed not suitable for work in numerous narrow stretches of lines and the depots in the Soviet capital. But for other cities, approval was obtained.

From the technical point of view, the T6B5 was a real revolution for Soviet tram workers. This car has reverted to double-stage suspension of the bogies, and disk brakes, but the main innovation is a semiconductor-based motor control system instead of the electro-mechanical booster unit on the T3. The new system differs completely from the old resistor-based one: it controls the torque of the main motors via changes of pulses in the electric current passing through them. If acceleration is needed, the pulses become more frequent and wide (in time axis on a waveform), the average current through the motors increases, and vice versa in the case of deceleration. The old resistor-based system performs the commutation of resistor groups for current regulation, a significant part of the electrical energy simply being dissipated in resistors without performing any usable work. This commutation is performed by an auxiliary servomotor and is not reliable due to the large number of mechanical contacts.

The T6B5 has a fully electronic thyristor-based control unit for producing current waveforms, and the mechanical contacts are used only for emergency circuit breaking. The noisy motor-generator on the T3 was replaced by a noiseless electronic transformer unit which converts 600V voltage to 24V for low-voltage devices. But the T6B5 does not become silent - the semi-conductor equipment produces a small amount of "crying" noise when the tramcar accelerates or decelerates.

The appearance of the driver's cab (black dash-board with buttons, lights and dials) resembles a scientific laboratory device rather than vehicle controls.

From the passenger's point of view, the T6B5 is also a step forward in comparison with the T3. The T6B5 has more seats and places for standing passengers; visibility from the interior is excellent due to the large windows; and passage to the doors is very comfortable, all the doors being located flush along the nearside. This door placement, however, requires the ends of the body to be wide, without taper, and thus restricts the tramcar's use in narrow places. In all other aspects, the T6B5 wins over all contemporary Soviet tramcars. But its exterior appearance resembles the LM-68 - the last Soviet cult tram. Of course, the Czech designers developed the T6B5 fully independently of their Leningrad colleagues, but it is interesting to note such a convergence in design of trams which were separated by a period of 15 years. If one wishes to be more exact, however, such a design was first implemented in 1972 on the T5 prototype mentioned above and four years later it was completed on the T6 series tramcars.

In 1985, the first 38 series-production T6B5s were built for the Soviet Union. They were delivered to Kiev, Kalinin and Barnaul. These tramcars were well received there and from 1987 Tatra launched the mass production of T6B5s; at the end of that year, T6B5s completely replaced T3s on the assembly lines. 365 tramcars were built in 1988 and 385 in 1989. Political and economic disruption, both in the Soviet Union and the Czechoslovak Republic, however, resulted in the cessation of T6B5 mass production. 217 tramcars were built in 1990 but the Soviet Union refused to purchase all of them; as a result 92 T6B5s were sold to North Korea and 37 to Sofia, Bulgaria. So the former USSR, North Korea and Bulgaria are now the only states where T6B5s operate. Only 23 T6B5s were built in 1991. There was a pause in production from 1992 until 1993. After this pause, Tatra built their last 101 tramcars and its tram department went into bancruptcy in 1997, resulting in there being uncompleted bodies and equipment; these were slowly assembled by the "Inekon Group" enterprise. The last of them were bought by Izhevsk in 2003. The Izhevsk tramway staff were as satisfied with this deal as they had been 20 years earlier.

A separate chapter in the T6B5 history was its assembly by "Yuzhmash" (an acronym for the Russian words meaning Southern Machinery) in the city of Dniepropetrovsk, Ukraine. In 1994, Tatra and Yuzhmash, which had been a military plant in the Soviet era, made an agreement about the assembly of T6B5s in Ukraine under the trademark "Tatra-Yug". Tatra delivered bodies and equipment and Yuzhmash assembled 27 T6B5s from these spare parts. After the delivery of spare parts ended, Yuzhmash tried to organize their own production using their own facilities. About two dozen bodies were constructed but only some of them were completed as T6B5s. By this time, Yuzhmash engineers, starting from the T6B5 design, developed their own tramcar, type K1. Some of the unfinished T6B5 bodies were equipped with K1 spare parts and the same plans exist for other incomplete bodies. But it is not correct to call these tramcars T6B5s; they are transitional variants between the T6B5 and K1. All Tatra-Yug tramcars (both using Czech and Ukrainian spare parts) are operated only by Ukrainian tramways. Some of them had even had no experience of the original T6B5 type in the Soviet era.

The original Tatras successfully conquered Soviet cities one by one. Of course, there were some difficulties at first, but the absence of complicated electro-mechanical units such as the T3 booster made the task easier. The Czech power electronics proved to be a reliable unit. For diagnosis and repair, however, the T6B5 requires highly qualified personnel, who were not to be found on some Soviet tramways. Sometimes T6B5s worked until their first serious (but not fatal) defect and thereafter were laid up due to incompetence amongst maintenance staff. So when the Ust'-Katav Wagon Building Plant started to produce KTM-8 tramcars, some of the operators turned their backs on T6B5s and sold them to other cities. Their purchasers were generally satisfied, however, despite the need for major repairs to the acquired T6B5s.

Without diminishing the advantages of the KTM-8, it has to be said that its design contains many items inherited from the KTM-5, the tramcar which was at least one and arguably two generations behind the T6B5. So the motto "Jedem Das Seine" was quite applicable to Soviet tram operators: some of them considered the T6B5 as their ideal, but others tried to escape from T6B5 delivery by hook or by crook. Amongst the latter were many operators who had failed with T3 tramcars. Minsk and Lipetsk, however, where T3s had never been operated at all, successfully launched the new T6B5s alongside their ancient RVZ-6 trams. This is yet further proof of the thesis that the human factor is decisive in any area and that success or failure in launching new machinery is only an indicator of personnel abilities.

In summary, it can be said that the T6B5 is comfortable for passengers and a reliable tramcar (in Moscow, the first prototype has been working from 1983 to the present time without the need for any serious repair). The T6B5 design has many features which were very advanced at the time of construction. On the other hand, to derive the fullest benefits and usage, the T6B5 requires highly qualified technicians in comparison with the previous generation of tramcars. But this is not a drawback - it is a common requirement for all advanced machinery of pioneering design.

In the author's opinion, the T6B5 is a very successful design and its Czech creators may be proud for it. But what a pity that, despite its relatively young age, its series production is now firmly and for ever in the past.