## 1 Introduction  
The ladder sleeper, which is a type of longitudinal sleeper with long beams in the longitudinal direction of the rail, was developed for the maintenance-labor saving of ballast tracks [1, 2]. The ladder sleeper's main components are two prestressed concrete (PC) beams, steel pipes, and post-tension PC end beams, and its structure differs significantly from that of a conventional prestressed concrete sleeper (PC sleeper) that is orthogonal to the rail. Since the ladder sleepers come into contact with the ballast on a continuous surface in the rail direction, they are superior in load distribution performance compared to PC sleepers that are in contact with the ballast discretely in the rail direction. As a result, the vibration transmission to the ballast caused by the train running can be reduced. As a result, the ladder sleeper is an effective track member not only for reducing structural vibration [3, 4, 5] and ground vibration [6], but also for improving the track's buckling stability [7, 8, 9]. So far, it has been laid mainly on conventional lines, subways, and light rail transit in Japan, China, Europe, etc. In addition, although the shape is significantly different from that of Japanese and Chinese ladder sleepers, it is also applied to light rail transit in Europe [10]. In recent years, research on the vibration mitigation characteristics of the ballasted ladder track with elastic elements (under-sleeper pad) has also been conducted [11].  
The ladder sleepers in Japan were researched and developed with the primary purpose of maintenance-labor saving for the ballast track, for which the regular track maintenance based on ballast compaction was indispensable and were put into practical use in 1999. In recent years, it has become increasingly common to apply it not only to general sections of ballast tracks, but also to structural boundaries that require frequent maintenance work, such as rail joints and expansion joints where impact loads are likely to occur, and level crossings where the support condition of the track changes, and so on. Structural boundaries between embankments, abutments, and bridge girders, as well as open culverts that cross railroad tracks, on the other hand, are frequent maintenance points. Even in such places, if a ballasted ladder track is laid, maintenance-labor saving is expected. However, in recent years, the number of installations in such places has tended to increase, but the reality is that there are few public data that quantitatively show the effect of labor saving in maintenance.  
It should be noted that the use of an under-sleeper pad (USP) and under-ballast mat (UBM) also can reduce the value of loads transmitted to the ballast [11; 12; 13; 14], but in this study, we only focus on the ladder sleeper without USP or UBM.  
In this study, we decided to verify the longitudinal level irregularity suppression effect at the structural boundary by the ladder sleepers. The three following points are focused on:  
1. To quantify the load distribution effect of ladder sleepers, full-scale model tests are conducted.
2. Carry out numerical experiments with a three-dimensional numerical analysis model, comparing ladder sleepers and conventional PC sleepers in terms of the pressure on the sleeper bottom plane.
3. Ladder sleepers are laid on the commercial line and long-term measurement of longitudinal level irregularity are carried out to verify the effect of suppressing the longitudinal level irregularity of the ballasted ladder tracks.