So I was surprised to see that there wasn't a thread dedicated to this particular subject, but if there is then I do apologise for missing it. That said I should make clear that in this thread I am not trying to start a discussion about what you prefer, but rather I would like to discuss the advantages and disadvantages of both loco-hauled units and multiple units (be that diesel or electric etc.) and which one is better for use in InterCity operations. The reasons for talking about that sector specifically will be touched upon shortly.
For loco-hauled units, there are advantages of easier maintenance since all of the train's power is centralised to one vehicle, and in the case of a failure said vehicle can be easily replaced with minimal shunting movements where passengers won't need to evacuate the train compared to multiple units where failure can result in the need to swap an entire unit, though this is somewhat mitigated if there is more than one unit formation, though that comes with the cost of overcrowding. Loco-hauled units are also flexible where if more power is needed you can always add another or a more powerful engine, and in every case because there is no undercarriage traction power the passenger saloon areas will almost always be quieter, especially in the case of DMUs.
For multiple units though, there are advantages of faster acceleration and higher power-to-weight ratio due to the multiple traction motors spread throughout the train, and this also gives them greater adhesion on gradients. The turnaround times are also faster than a loco-hauled unit because all that is required is to switch cab ends. Two coupled units also make it easier to split trains mid-journey due to not needing to find another locomotive to take the coaches that are left behind at a station. Multiple units also have lighter axle loads and thus will be able to travel on lighter tracks where heavy locomotives may be banned. This also reduces track wear over time thanks to the lighter axle loads, which can be a huge advantage in the long term maintenance costs.
With the advantages and disadvantages in mind it seems like both types of trains are beneficial in different ways and are suited to different types of train usage. For freight trains which don't need the fast turnaround times or particularly fast acceleration, the perks of a multiple unit are very much moot and therefore loco-haulage is more suitable due to the greater flexibility and easier maintenance. In fact having a multiple unit would be detrimental due to the fact that the wagons would lose space by having traction motors on the wheel bogies. Meanwhile, the opposite is very much the case for suburban commuter rail where fast turnaround time is needed, quick acceleration is key to the consistent stop-start service patterns, and where multiple units can be coupled together and split later on in a journey without spending too much time preparing a new loco or signalling new movements for one at a station on a busy route. Multiple units are very much ideal for such operations as well as light metro rapid transit systems.
But for InterCity style operations, this is where I'm not so sure. For one, InterCity operations do not require the same rapid acceleration rates as commuter style operations, but they may operate on otherwise very busy routes such as the West Coast Main Line and so still benefit from faster acceleration. But otherwise neither option seems truly better than the other. Turnaround times will not be as fast as commuter trains, and even though some loco-hauled units will require shunting movements, there are push-pull units such as the Class 68 and MK5 coaches for TPE that share the advantage of fast turnaround times for multiple units but also come with the setback of a fixed formation which means it won't be as easy to switch locos as it would in a non push-pull unit. That said, modern locos are very reliable and so breakdown concerns aren't demonstrably big enough to warrant not using fixed-formation loco-hauled units. Furthermore modern powerful locomotives can easily match the power of a multiple unit on steep inclines.
Ultimately though I am still not quite so sure. What exactly determines whether an InterCity operator will use loco-hauled units or multiple units for their operations? It is very clear that both have their unique advantages, much to the point that the Nova fleet for TPE was a mixture of loco-hauled and EMUs, and why many European operators such as SBB and OBB use a mixture for their long distance intercity operations. Even high-speed rail has a mixed usage such as all SNCF TGV stock and the Renfe Class 100 and 102s being a loco-hauled unit with power cars on each end, and the Siemens Velaro, Alstom AGV and Zefiro fleets being high-speed EMUs. Tilting trains may be the only time multiple units have an advantage since the Pendolino tile of 8 degrees is greater than the Avelia Liberty coach tilt of 6.3 degrees, but then the differences there may not be related to the traction distribution at all, and even if it were tilting trains are a very specific niche market anyway and thus doesn't really have a huge impact on the entire debate in my opinion.
So ultimately, what exactly is it that determines whether or not an operator will use a loco-hauled unit or multiple unit stock for long-distance intercity operations? Does line capacity play a part in it? Does the power source (ie. diesel or electric) also play a part in the choice? Or is it quite simply down to operator preferences? Do operators simply choose what units they want based on things such as price or preferences?
For loco-hauled units, there are advantages of easier maintenance since all of the train's power is centralised to one vehicle, and in the case of a failure said vehicle can be easily replaced with minimal shunting movements where passengers won't need to evacuate the train compared to multiple units where failure can result in the need to swap an entire unit, though this is somewhat mitigated if there is more than one unit formation, though that comes with the cost of overcrowding. Loco-hauled units are also flexible where if more power is needed you can always add another or a more powerful engine, and in every case because there is no undercarriage traction power the passenger saloon areas will almost always be quieter, especially in the case of DMUs.
For multiple units though, there are advantages of faster acceleration and higher power-to-weight ratio due to the multiple traction motors spread throughout the train, and this also gives them greater adhesion on gradients. The turnaround times are also faster than a loco-hauled unit because all that is required is to switch cab ends. Two coupled units also make it easier to split trains mid-journey due to not needing to find another locomotive to take the coaches that are left behind at a station. Multiple units also have lighter axle loads and thus will be able to travel on lighter tracks where heavy locomotives may be banned. This also reduces track wear over time thanks to the lighter axle loads, which can be a huge advantage in the long term maintenance costs.
With the advantages and disadvantages in mind it seems like both types of trains are beneficial in different ways and are suited to different types of train usage. For freight trains which don't need the fast turnaround times or particularly fast acceleration, the perks of a multiple unit are very much moot and therefore loco-haulage is more suitable due to the greater flexibility and easier maintenance. In fact having a multiple unit would be detrimental due to the fact that the wagons would lose space by having traction motors on the wheel bogies. Meanwhile, the opposite is very much the case for suburban commuter rail where fast turnaround time is needed, quick acceleration is key to the consistent stop-start service patterns, and where multiple units can be coupled together and split later on in a journey without spending too much time preparing a new loco or signalling new movements for one at a station on a busy route. Multiple units are very much ideal for such operations as well as light metro rapid transit systems.
But for InterCity style operations, this is where I'm not so sure. For one, InterCity operations do not require the same rapid acceleration rates as commuter style operations, but they may operate on otherwise very busy routes such as the West Coast Main Line and so still benefit from faster acceleration. But otherwise neither option seems truly better than the other. Turnaround times will not be as fast as commuter trains, and even though some loco-hauled units will require shunting movements, there are push-pull units such as the Class 68 and MK5 coaches for TPE that share the advantage of fast turnaround times for multiple units but also come with the setback of a fixed formation which means it won't be as easy to switch locos as it would in a non push-pull unit. That said, modern locos are very reliable and so breakdown concerns aren't demonstrably big enough to warrant not using fixed-formation loco-hauled units. Furthermore modern powerful locomotives can easily match the power of a multiple unit on steep inclines.
Ultimately though I am still not quite so sure. What exactly determines whether an InterCity operator will use loco-hauled units or multiple units for their operations? It is very clear that both have their unique advantages, much to the point that the Nova fleet for TPE was a mixture of loco-hauled and EMUs, and why many European operators such as SBB and OBB use a mixture for their long distance intercity operations. Even high-speed rail has a mixed usage such as all SNCF TGV stock and the Renfe Class 100 and 102s being a loco-hauled unit with power cars on each end, and the Siemens Velaro, Alstom AGV and Zefiro fleets being high-speed EMUs. Tilting trains may be the only time multiple units have an advantage since the Pendolino tile of 8 degrees is greater than the Avelia Liberty coach tilt of 6.3 degrees, but then the differences there may not be related to the traction distribution at all, and even if it were tilting trains are a very specific niche market anyway and thus doesn't really have a huge impact on the entire debate in my opinion.
So ultimately, what exactly is it that determines whether or not an operator will use a loco-hauled unit or multiple unit stock for long-distance intercity operations? Does line capacity play a part in it? Does the power source (ie. diesel or electric) also play a part in the choice? Or is it quite simply down to operator preferences? Do operators simply choose what units they want based on things such as price or preferences?
